An Introduction to Geology, illustrative of the general structure of the earth; comprising the elements of the science, and an outline of the geology and mineral geography of England

Introduction

TO GEOLOGY, ILLUSTRATIVE OF THE GENERAL STRUCTURE OF THE EARTH; COMPRISING THE ELEMENTS OF THE SCIENCE, AND AN OUTLINE OF THE GEOLOGY AND MINERAL GEOGRAPHY OF ENGLAND: By ROBERT BAKEWELL. " A knowledge of our subterranean wealth would be the means of furnishing greater opulence to the country than the acquisition of the mines •f Me_ico and Peru." President of the Board of AipicultuTt. LONDON : PRINTED FOR J. HARDING, 36, ST. JAMES'S STREET. Price \As. 1813. Ifunted kij Himuni lay tor una Oo. _>_tot> _._....-, Luuuun.

Preface

. In tracing tbe progress of knowledge, we may frequently observe that the cultivation of particular branches of science at certain periods was determined by causes which had little connection with their intrinsic utility. The authority of eminent names, and of fashion and caprice, influences mankind in philosophy as well as on all other subjects. But, independent of accidental causes, there are certain leading objects in the universe, which, as nations advance in civilization, seem naturally to direct their attention to certain sciences in succession. The brilliancy of the sun, moon, and planets, their various motions, and connection with the changing seasons, would first arrest the attention ofthe rude philosopher; nor need we won der that he soon began to regard them as a 2 endowed

PREFACE.

. endowed with life and intelligence, and at tributed to them a n^sterious power over human affairs: thus the heavenly orbs be came the objects of religious adoration.; and curiosity, hope, and fear, lent their aid to the eariy cultivation of astronotny. Mathematics and mechanical philosophy are so intimately connected with astro nomy and the most useful arts, that they naturally claimed the second place among the early sciences. The branches of philosophy which com prise a knowledge of the physical qualities of matter, or such as are perceptible by the senses, follow next; and at a later pe riod chemical philosophy, or that science which endeavours to ascertain the elementary substances of which all material ob jects are composed. In the order of suc cession, mineralogy and geology are the last ofthe natural sciences ; for though an acquaintance with the earth is more im portant to man than a knowledge of the distant V

PREFACE.

. distant parts of the universe* yet previous to the cultivation of the other sciences, and in particular of chemistry, our know ledge of the mineral kingdom could not advance beyond that ofthe rudest periods. Thus we find that notwithstanding the pre cious metals, and many of the mineral treasures which the earth contains, have been the objects of insatiable cupidity in every age, yet, till the present day, almost nil that was known of mineralogy was con fined to uneducated working miners. In looking over the pages of history, we shall observe that the nations of an tiquity had not advanced beyond a limited acquaintance with astronomy and mathematical philosophy. In modern Eu rope all the other sciences, geology and mineralogy excepted, have been success fully cultivated, and their progress has been astonishingly rapid ; but till about the middle of the last century the structure of the earth had scarcely engaged

PREFACE.

. VI gaged the attention of philosophers. Near that time, Lehman, the German, first observed that there are certain rocks which occupy the lowest relative situation in different countries, and that these rocks contain no organic remains : hence he gave them the name of primary, and established a division between them and the rocks by which they are covered, in which the remains of animals or vege tables frequently occur : the latter he called secondary. In our own country, the Re verend Mr. Michell was the first person who appears to have had any clear views respecting the structure ofthe external parts of the earth : they were made public in a valuable paper on the cause of earthquakes in the Philosophical Transactions, 1759. About twenty years afterwards, Mr. John Whitehurst published his " Inquiry into the original State and Formation ofthe Earth." His observations were principally confined to the rocks and strata of Derbyshire. Independently

PREFACE.

. pendently of its speculative opinions, this work was highly valuable, as an attempt to describe the geology of a district from actual examination. The great variety of origi nal information itcontained,and its general accuracy, will remain a lasting monument of the writer's industry and ability. Mr» Whitehurst, however, fell into the same error with the celebrated Werner in Sax-* ony, an error to which the first cultivators of geology were particularly exposed, that of drawing general conclusions from par tial observations, and forming universal theories from a limited number of facts. Though Mr. Whitehurst's book was fa vourably received, yet till the beginning of the present century geological pur suits made little progress in England. On the continent, the researches of Saussure, Pallas, Werner, St. Fond, Dolomieu, and others, had before this time produced a powerful interest, and brought into the field many active and enlightened inquirers.

PREFACE.

. rers. The first general impulse given to the public taste for geological investigations in this country was produced by Professor Playfair's luminous and eloquent illustra tions ofthe Huttonian theory. The lead ing feature of this theory, that all rocks or strata have been formed or consolidated by central subterranean fire, was ably and very warmly opposed; and much per sonal feeling and many adventitious circumstances were associated with the con test, not highly honourable to philosophy, but well calculated to keep the attention of the disputants alive to those appearances in nature which favoured or opposed their different theories. The interest excited by the brilliant dis coveries lately made in chemistry has been highly favourable to the progress of geology, by developing new views respecting the objects in the mineral kingdom, and cre ating a desire to be acquainted with the situations in which they occur.

Hence a taste TREFACE. IX taste for such inquiries is more prevalent among the intelligent classes of society in this country than at any former period; and in the course ofmy profes sional pursuits, and the geological lec tures I have occasionally delivered, I have repeatedly heard the want of an intelligible introductory treatise on geology much re gretted, by persons who lamented their in ability to comprehend the ' Geognosy' of Mr. Jameson, and their equal inaptitude of attention to the polemical controversies of M. De Luc. To both these gentlemen geology is under considerable obligations; and if their elementary works have failed in exciting general interest, it is owing, I conceive, to an over-strained attachment to their particular theories, or to the tech nical obscurity in which those theories are involved.

He who attempts to make a scientific subject familiar, runs the risque in this country _R__ ACfi. X Country of being deemed superficial : a plentiful share of dullness, combined with a certain degree of technical precision, are regarded as essential proofs of profundity. By prescriptive right long established in these realms, dullness and pedantry guard the portals of the temple of Science, and command those who enter to avert their eyes from whatever can elevate the imagination, or warm the heart, and to look at nature through a sheet of ice. In com pliance with their authority, writers of in troductory treatises have generally thought it necessary to avoid that felicity in the familiar illustration of scientific subjects so conspicuous in some of the elementary works of our neighbours. Without venturing to depart too far from established usage, I have endeavoured to render geology more intelligible, by avoiding as much as pos sible theoretical and technical language, and by introducing a simple arrangement suited

PREFACE.

. suited to the present state of our know ledge. The local illustrations from va rious parts of our island, with the draw ings, sections, and map in the present volume, will I trust facilitate the study of geology, and prove particularly accept able to those who are entering on these __> inquiries: at the same time, I flatter myself with the hope, that the original information this work contains respecting the geology and natural history of England, will secure it a candid reception. Several have been deterred from the study of geology by the supposed difficulty of learning its attendant science, mineralogy ; but an acquaintance with the nice distinctions made by many modern mine ralogists is not necessary to gain a know ledge of the structure and arrangement of the great masses of matter that environ the globe, nor of the substances of which they are composed. The number of simple

PREFACE.

. pie minerals which form rocks and strata is small; and when the student is well acquainted with their physical and che mical properties, their external characters, and more common modes of intermixture and combination, he is prepared to com mence an examination of nature for him self with satisfaction and advantage. The pedantic nomenclature and frivolous di stinctions recently introduced into minera logy, may gratify vanity with a parade of knowledge ; but they are unconnected with objects of real utility, or with any enlarged views of nature. On hearing the various names which mineralogists give to the same substance, and observing the avidity with which each new name is seized, 'as if it conveyed a hidden charm, the uninitiated might sup pose that he was "journeying in the land of Shinar," and had fallen in company with a set of masons fresh from the tower of Babel,

PREFACE.

. XIII bel, each one calling the same stone by a different name, and glorying in his absurdity. Such frivolities disgust men of sense with the study of an important and interesting science, a science that has for its immedi ate object the structure of the planet which the author of nature has destined for our abode, and an acquaintance with the situa tion of its various mineral productions, sub servient to the wants or enjoyments osman in civilized society. The advice of Cicero to the cultivators of moral science applies with peculiar force to the geologists and mineralogists of the present day. ' In these natural and lau dable pursuits, two errors are particularly to be avoided; the first, not to confound those things of which we are ignorant with those we know, or rashly to yield our as sent without due investigation; the second, not to bestow too much labour and study on obscure, intricate, and unprofitable subjects.'

PREFACE.

. XIV jects.' ' In hoc genere et naturali et ho nesto duo vitia vitanda sunt : unum, ne in cognita pro cognitis habeamus, hisque temere assentiamur (quod vitium effugere qui volet adhibebit ad considerandas res et tempus et diligentiam). Alterum est vitium, quod quidam nimis magnum stu dium multamque operam in res obscuras atque difficiles conferunt, easdemque non ntcessarias.' — Cic. Offic. i. 6'. From Cuvier 's large work, entitled " Recherches sur les Ossements fossiles des Quadrupedes," which has recently arrived in this country, it appears that the gypsum of Mont martre near Paris, in which are found bones of birds and qu: drupeds, does not rest immediately upon chalk, as stated page 179, but upon the eartliy limestone (calcaire grossiere) over the chalk. — See the Appendix to the present work, No. II. where the reader may find a correct account of the strata near Paris. CON CONTENTS.

INTRODUCTION.

. The objects of the science denominated Geolony. — Tbe shape of the earth and the other planets affected by the respective velocities of their rotary motion. — Density of the earth. — Theoretical division of rocks and mountains into primary, intermediate, and secondary. —Arrangement of the mineral districts of England — The present continents once covered bv the ocean : existing pi-oofs of this in England, Spain, South America, and various parts of the world. — Fossil remains of shellfish and zoophytes imbedded in rocks, sometimes intermixed with those of quadrupeds. — Inferences respecting the former condition ot the globe - - - 1

Chapter I. On The Structure And Composition Of Rocks

Terms employed in Geology. — Rocks, Simple or Compound: Varieties of their internal structure. — External structure of Mountain Masses: — Indeterminate, Stratified, Columnar, Tabular, and Globular: — the three latter the Effects of Crystallization en masse. — Tabular Structure erroneously confounded with the Stratified. — Elementary Substances — Simple Minerals of which Compound Jtocks arc formed - - - - 21

Chapter II. On The Position And Arrangement Of Rocks And Strata

Different positions explained. — Relative ages of rocks determined in -ome instances by their position. — Appearances presented by stratified parallel rocks, illustrated by a section of the country from Sheffield to Castleton. — Excavation of Valleys. — Valleys formed *or CONTENTS. by the dislocation of Rocks and Strata. — Beds of Gravel : opinion* respecting their formation. — Transportation of blocks of Granite to distant countries, how effected. — Bones of large quadrupeds transported without injury by similar operations. — Hills and Moun tains, observations respecting their declivities - - i'l

Chapter III. On Granite And Other Primary Rocks

Geological Classification. — Granite rocks: their structure and appearance. — Supposed stratification of Granite the effect of crystalline arrangement. — Granitic mountains of Europe and America. — Situation of Granite Rocks in England. — -Rocks of Gneiss and Micaceous Schist described. — Crystalline Lime-stone. — Formation of Lime by animal secretion. — Origin of Calcareous Rocks. — Hornblende Rock. — Serpentine Rock.— Verde Antique. — Beautiful Serpentine fiom Anglesea. — Anomalous Rocks - - 56

Chapter IV. On Slate, Mountain Lime-stqne, And Other Inter- Mediate Or Transition

ROCKS. Organic remains in this class of Rocks. — Argillaceous Schistus or Slate. — Varieties of Slate. — Gray-wacke or Coarse Slate. — Sub-. crystalline Lime-stone or common Marble — Amygdaloid of Derbyshire. — Opinions respecting its formation. — General view of Primary and l'ransition Mountains - - - 84

Chapter V. Basaltic Rocks, Porphyry, And Sienite

Position of these Rocks, their gradation into each other, and resemblance to volcanic products. — Sienite, Green-stone, Basalt, Amygdaloid, and Porphyry described Neglect of Porphyry by modern arc'itects.— Basaltic Rocks of Sicily and Auvergne. — Columnar and Globular Basalts, experiments explaining their formation. — Basaltic Columns of Ireland, — Basaltic Rocks extending in the same parallel of Longitude. — Speculations of different Geologists. — Theory of Werner. — Observations of Sir G. Mackenzie in Iceland.— Remarkable appearance of Basalt at the Clee Hills iu Shropshire and in the county of Durham, illustrating the origin of Basalt - - - -97 CHAP- CONTENTS. XVII

Chapter VI. On Secondary Rocks Containing Rock Salt, Coal, And Iron Stone

Secondary Strata limited in their extent. — Opinions respecting their universal formation. — Red Sand Rock on the western side of England. — Rock Salt and Salt Springs. — Remarkable repositories of Ruck Salt. — Brown and Black Coal. — Coai Strata, their peculiarities described : — position of Coal Strata. — Dislocations or Faults. — Geological relations of Coal Strata. — Observations on the modes of searching for Coal. — Iron Stone accompanying Coal. — Precautions necessary in the establishment of Iron Founderies. — Varieties of Coal. — Plumbago. — On the origin and formation of Coal - - - - - 129

Chapter VII. On The Upper Secondary Rocks, And The Strata Con- Taining

REMAINS OF RIVER FISH AND QUADRUPEDS. Depth of the Secondary Strata lying between the surface and primary rocks. — Earthy and Magnesian Lime-stone. — Portland Stone, —Chalk Rocks. — On the formation of Flint and Chalk. — Beds of Gypsum; opinions respecting their formation. — Gypsum of Derbyshire and Nottinghamshire. — Bones of quadrupeds, and Shells of river fish in the strata over chalk.— Alternation of marine and fresh-water Shells near Paris Geological inferences. — Fresh-water Lakes of America and Aotient Europe - - 164

Chapter VIII. On The Decomposition Of Mountains, The Formation Of Soils And

ALLUVIAL PRODUCTS. Proofs of the diminution of Mountains. — Elementary agency.— Final causes. — Formation of Soils. Mixture of different Earths necessary to form good Soil. — Effects of Lime. — Alluvial depositions from Alpine Countries and Secondary Rocks. — Peat Moors, and Subterranean Forests ... 187

Chapter IX. Mineral Dykes And Metallic Veins

Dykes intersecting primary and secondary Mountains. — Their structure, contents, and extent. -Dislocations of the Strata caused by b Dykes. CONTENTS. Dykes.— Structure of Metallic Veins.— Their depth and thickness. —Substances found in Veins.— Pipe Veins.— Fluor Mine at Castle ton.—Position of Veins.— Extent of Veins— Opinions respecting the manner in which Metallic Veins were filled.— Theory of Wer ner proved to be erroneous, by striking appearances in different English Mines — Facts elucidating the Formation of Metallic Veins - - "~ - - 20T

Chapter X. On Earthquakes

aND VOLCANOES. Phænomena preceding and attending Earthquakes ; their effects on Water,— on Land : — their connection with distant Volcanoes ; more severely felt in mountainous Countries. Indications of their Cause. — Volcanoes. — Phænomena preceding an Eruption. — Duration and intervals of Volcanic Eruptions. — Submarine Volcanoes. —Number of active and dormant Volcanoes. — Source of Volcanic Fire deep below the surface. — Opinions respecting it. — Hot- Springs. — Volcanic products — Solid, Fluid, and Volatile. — Compact and porous Lava.— Pumice, Obsidian, Volcanic Tufa, Puzzolano. — Inflammable Substances emitted from Volcanoes 230

Chapter XI. An Outline Of The Geology Of England

The surface of England naturally divided into three districts, her* denominated the low district, the middle district, and the alpine district. — Situation ofthe low district (257). — Nature of the Rocks and Strata. — Their succession, thickness, and extent — Organic remains peculiar to this district — Situation of the middle district (264) — containing Iron-Stonc,'Coal, and Rock Salt. — Alum Rocks nt ar Whitby. — Rocks and strata of the middle and low district described in succession as they occur in crossing the Island from Hull to Liverpool, and in other directions — The Alpine district subdivided by the Bristol Channel and the plains of Cheshire into three mountain ranges, here denominated the northern, the Cumbrian, and the Devonian (2?5) — Extent and direction of the northern range. — Height ofthe Mountains in Cumberland, Yorkshire and Derbyshire.—.

Nature and succession of the Rocks Thickness of the Strata iu Derbyshire, determined by actual admeasurement.— Mineralogical account of the Rocks of Charnwood Forest in Leicestershire (281). — Malvern Hills in Worcestershire. — Cambrian Range (294). — Mountains of North and South Wales. — Colum- CONTENTS. XIX —Columnar Structure of Cader Idris.— Rocks in Radnorshire — Devonian Range (300). — Height, extent, and direction of the Hills. — Nature of the Rocks and Mineral products. — Ho^Springs in England. — Indications of the former agency of subterranean Fire.— General observations on the advantages to be derived from a more perfect acquaintance with the Mineral productions ef our own Country. - ... 255

Chapter XII. Conclusion

On the Crystallization of Mountain Masses. — Variation of Polar Magnetism. — Geological Theories. — Formation of Planetary Bodies. — Appearance of new Stars. — Inferences and Conclusion.

Appendix

. No. I. Situations in which Metallic Ores arc found. — II. An account of the strata round Paris. — III. On the recent formation of Flint. — IV. Remarks on a passage in Sir Humphry Davy's Lectures. — V. Temperature of Hot Springs.— VI. Cobalt mine at Alderley Edge in Cheshire \ - - - - 331 VOCABULARY OF TERMS - - 349 THE AOTHOlt'S NOTICE • - DIRECTION FOR PLACING THE PLATES. Plate 1 to face page 44 2 - - - 48 3 (Basaltic columns, &c.) - 97 4 (Map of England) - - 255 5 (Cader Idris) to face the Title. OUTLINES of GEOLOGY.

INTRODUCTION.

. The objects of the science denominated Geology. — The shape ofthe earth and the other planets affected by the respective velocities of their rotary motion. — Density of the earth. — Theoretical division of rocks and mountains into primary, intermediate, and secondary. — Arrangement of the mineral districts of England. — The present continents once covered by the ocean : existing proofs of this in England, Spain, South Ameri ca, and various parts ofthe world. — Fossil remains of shells sh and zoophytes imbedded in rocks, sometimes intermixed with those of qua drupeds. — Inferences respecting the former con dition ofthe globe. VV hen we survey the globe we inhabit, where the solid parts of the surface are not covered by soil or loose materials, we observe rude masses of stone or rock lying in confusion upon each other ; or should we perceive some regularity in their position extending b over OBJECTS OF THE SCIENCE over a limited district, we soon lose sight of it again by the intervention of other rocks.

In this department of nature, all appears vast, unshapen, and chaotic, and the exquisite skill which is displayed in the organization of the minutest animal or vegetable eludes our observation when we examine the solid pavement of our planet, In other parts of the universe also, the first view presents us with the same apparent want of order and design. The shepherd who discovered that the planets were not fixed in the heavens, could observe them wandering backwards and forwards amongst the stars ; but it was impossible for him to trace any re gularity in their courses, much less could he perceive the harmonious simplicity of their movements which subsequent discoveries have demonstrated. Let us not then hastily con clude that the mineral kingdom is composed of a chaotic mass of rocks and stones extending without any order to an indefinite depth, but let us more wisely endeavour to ascertain the means by which we may acquire better information.

Were these means bounded by the DENOMINATED GEOLOGY. the power of man to penetrate the surface of the earth, our knowledge of it must be very limited and imperfect ; but natural operations have greatly facilitated our inquiries, and in various situations have raised up or laid bare a considerable depth of the solid covering of the globe. The knowledge of the structure, composition, and arrangement of the materials which form mountains, rocks, or strata, constitutes the first part of the science called geology. In the second part, we may include the direction, structure, and extent of the mineral dykes and metallic veins by which they are inter sected. In the third part, the changes which are taking place on the surface of the globe by the agency of inundations, earthquakes, and volcanoes. There is a fourth part which may be styled speculative geology, or an in vestigation of the causes that have probably operated in the formation of rocks and moun tains, and also those by which the revolutions ofthe earth's surface have been subsequently effected.

Nor is this part, as some assert, entirely useless; the advocates of particular b 2 systems FORM AND DENSITr systems have engaged in an active examination of nature to support their opinions, and have " compassed sea and land to gain proselytes :" thus numerous facts have been disco vered, with which we should not have been acquainted had they remained idle in their stu dies. It may, however, be doubted whether they have not sometimes been insensibly in duced to close their eyes on other facts that opposed their favourite theories. The earth is now well known to be one of those globular bodies called planets, that revolve round the sun in orbits nearly circu lar, and in stated periods of time which bear a certain ratio to their respective distances from it. They turn round their axes with different degrees of velocity, and this motion appears to have had considerable influence on their external shape by enlarging their equatorial diameters.

In the planet Jupiter, the velocity of the equatorial parts is more than 400 miles per mi nute, whilst in the same time the equatorial parts of the earth have moved only 17 miles. A difference between the polar and equatorial OF THE EARTH. rial diameter of Jupiter is perceptible with a telescope that has a distinct magnifying power of 100 times, and it is ascertained to be as 12 to 15. The equatorial diameter of the earth exceeds its polar 37 miles, or is only as 230 to 229- The relative density of the sun, the earth, and of the other planets, is estimated by the attractive force which they exert on each other as they move round their common centre of gravity. The absolute density or the quantity of matter contained in the earth, compared with an equal bulk of any known substance, may be nearly determined by the attractive force which any given mass of matter exerts upon a plummet (when sus pended in its vicinity) to draw it from a vertical line.

This will be proportional to the absolute quantity of matter in that mass compared with that of the earth. By this method, it has been found that the mean density of the earth compared with that of an equal bulk of water is as five to one, which is nearly twice the weight of most of the rocks and stones on its surface. b 3 Hence THEORETICAL DIVISION. Hence it may be inferred that the interior part of the earth is solid ; or, if it be ca vernous, the solid matter must possess great density. It is not improbable that iron nearly in a metallic state may be one of the constituent parts of the central mass, and to this it may owe its magnetic polarity. Dr. Halley has written a very ingenious paper (in the Philosophical Transactions) to prove that the earth is a hollow sphere, in which there is inclosed a central magnetic globe, and by the motions of this globe the variations of the magnetic needle are pro duced.

It is evident that we have no means of verifying or invalidating hypotheses respecting the nature of the central parts of the globe. The matter thrown up from vast and unknown depths by subterranean fires is si milar to that of many rocks on the surface ; but we know not what changes it has under gone, or what substances were separated from it by fusion. The rocks and mountains composing the solid parts of the earth's surface have been divided into different classes by geologists ; nor OF ROCKS AND MOUNTAINS. 7 nor are the divisions altogether abitrary. Certain mineral productions, as metallic ores and coal, are confined to certain rocks, in some of which they frequently occur, in others they are never found. There are also distinct characters peculiar to each class. The names which the classes have received are not free from the objection of being founded on hypothetical principles ; but as they have been generally adopted, it may be proper to retain them in a restricted sense, without any reference to theory, particularly as those who are entering on the study of geology will sind these names in constant use in every work relating to the science, and in the books of many modern travellers.

These divisions include primary rocks, intermediate or transition rocks, seco/idary rocks, alluvial ground, and volcanic products. Primary rocks have been so called because no organic remains have been found in them : hence it is supposed they were formed prior to the creation of animals or vegetables. They are extremely hard, and the substances of which they are composed are crystallized. b 4 They PRIMARY ROCKS. They form the lowest part of the earth's sur face with which we are acquainted ; and they not only constitute the foundation on which the other rocks rest, but in many situations they pierce through the incumbent rocks and strata, and form also the highest mountains in alpine districts. We are not to conclude, when we see a mountain or range of mountains bounded by a plain, that they terminate at their apparent bases.

On the contrary, they dip under the surface at angles more or less inclined, stretching below the lower grounds and lesser hills, and often rise again in remote districts. That primary rocks environ the whole globe will not admit of direct proof; but, from their frequent occurrence in moun tainous districts in the most distant parts of the world that have been examined, we may infer that some of the rocks of this class con stitute the foundation rock of every country. We have no means of ascertaining that the similar rocks of distant districts were formed at the same time, nor can we be certain that the rocks called primary have not once contained INTERMEDIATE AND SECONDARY ROCKS. tained organic remains that were destroyed during the process by which they acquired their present crystalline structure.

We may, however, with apparent probability, infer that their formation was prior to the existence of animals or vegetables on our planet in its present state, because the intermediate rocks which cover them contain the organic remains of zoophytes, or those animals which are considered as forming the first link in the chain of animated beings. The intermediate rocks have, on this account, been called by the Germans transition rocks, from the supposition that they were formed when the world was passing from a chaotic to a habitable state. These rocks are less perfectly crystallized than the former division, and contain, like the secondary, me chanical depositions. They separate the primary from the secondary rocks, and partake of the nature of both. The primary and transition rocks contain few saline or inflammable fossils ; but they are the repositories of metallic ores, which are not often found in the third division, or VOLCANIC PRODUCTS. or what are called the secondary stratified rocks, in many of which numerous remains of vegetables and animals occur.

This division contains sand-stone, coal, stratified lime stone, chalk, &c. Pebbles and water-worn fragments of rocks belonging to the former divisions are commonly found in many of the secondary rocks : hence it is inferred that they have been formed at a later period, and hence this class receives its name. Alluvial ground is the land formed from the ruins of other rocks by the agency of water ; it consists of gravel, clay, &c. Volcanic products are the substances ejected from volcanoes, or formed by the agency of subterranean fires. They constitute no inconsiderable portion of the earth's surface, though they are scarcely noticed by some geologists. Each of these classes has its appropriate mineral productions; and it would be as vain to seek for common coal in the primary and transition rocks, as to expect statuary marble or metallic veins in the upper secondary strata or in alluvial ground.