The Geological Story briefly told. An introduction to geology ... With numerous illustrations

Front matter

THE GEOLOGICAL STORY BRIEFLY TOLD. &n IntrooiLction to Ceologp FOR THE GENERAL READER AND FOR BEGINNERS IN THE SCIENCE. BY JAMES D. DANA, LL.D., AUTHOR OF "A MANUAL OF GEOLOGY," " TEXT-BOOK OF GEOLOGY," "CORALS AND coral islands," works on mineralogy, etc. WITH NUMEROUS ILLUSTRATIONS. NEW YORK AND CHICAGO : IVISON, BLAKEMAN, TAYLOR, AND COMPANY. I87S. Copyright, 1875. Br IVISON, BLAKEMAN, TAYLOR, & CO. University Press: Welch, Bigelow, & Co., Cambridge. PEEFATOET SUGGESTIONS. QEOLOGY is eminently an out-door science ; for strata, rivers, oceans, mountains, valleys, volcanoes, cannot be taken into a recitation-room. Sketches and sections serve a good purpose in illus trating the objects of which the science treats, but they do not set aside the necessity of seeing the objects themselves. The reader who has any interest in the subject should therefore go, for aid in his study, to the quarries, bluffs, or ledges of rocks in his vicinity, and all places that illustrate geological operations.

At each locality accessible to him he should observe the kinds of rocks that there occur ; whether they consist of layers or not ; and their positions, whether the layers are horizontal, — the positon they had when made ; or whether inclined, — a slope in the beds being evidence of a subterranean movement like that which takes place in mountain-making. Geology teaches that much the larger part of the rocks that con sist of layers were made through the action of water ; and if such rocks are accessible, it is well, after learning the lessons of the book, to look among them for evidence of this mode of origin, either in the structure of the layers, in the nature of the material, in markings within the beds, or in the presence of relics of aquatic life, such as shells, bones, etc. If some of the layers in a bluff consist of sand stone, others are pebbly, others clayey, and one or more are of limestone, the kinds of changes in the waters that took place to pro duce so varied results should be made a point for investigation.

If an excavation for a cellar is opened near an accustomed walk, it is best to look at the sections of the earth or sands thus made ; for these sands are very often in layers, and, in that case, they bear evidence that there even the loose material of the surface had been arranged by water, either that of the ocean or that of a river or lake. When the layers contain fossils, a collection should be made for study ; for they show what living species populated the waters or land when the rocks were forming ; and in the height of a single bluff there may be records thus made of several successive popula tions differentfrom one another. If a beach or a cliff along the ocean is accessible, the action of the waves in their successive plunges may be watched to great ad vantage ; for they are thus grinding up the stones and sands of the beach, and eroding and undermining the cliff.

Part I

— ROCKS, OR WHAT THE EARTH IS MADE OF. I. Minerals 3 1. Consisting of Silica ........ 3 2. Silicates 5 3. Carbon and Carbonates . 8 4. Ores 10 II. Kinds of Rocks 13 III. Structure of Rocks 19 PART H. — CAUSES IN GEOLOGY, AND THEIR EFFECTS. I. Making of Rocks 23 1. Ways in which Plants and Animals have contributed to Rock making ......... 27 1. Making of Limestones ...... 27 2. Making of Siliceous Rocks or Masses . . . .35 3. Making of Peat-beds 40 2. Quiet Work of Air and Moisture 42 3. The Work of Winds 44 4. The Work of Fresh Waters 46 5. The Work of the Ocean 50 6. The Work of Ice 57 7. The Work of Heat in Rock-making 63 1. Through Expansion and Contraction . . . .63 2. Through Fusion : Volcanoes 64 3. Solidification, Metamorphism, and Formation of Veins . TABLE OF CONTENTS. X II. Making of Valleys 76 III.

Making of Hills and Mountains and the attendant Effects 80 1. Mountains made by Igneous Ejections 80 2. Mountains and Hills produced by the Erosion of Elevated Lands 81 3. Mountains made by Upturnings and Flexures of Rocks, and Bendings of the Earth's Crust 83

Part III

— HISTORICAL GEOLOGY. Subjects and Subdivisions 96 I. Archæan Time 106 1. Distribution 107 2. Rocks 109 3. Life Ill II. Paleozoic Time 113 1. Silurian Age, or Age of Invertebrates . . . . .113 1. Lower Silurian ....... 115 2. Upper Silurian 129 3. Observations on the Silurian Age .... 134 2. Devonian Age, or Age of Fishes . . . . . .137 1. Rocks 137 2. Life 139 3. Mountain-making ....... 148 3. Carboniferous Age, or Age of Coal Plants .... 149 1. Rocks. — Coal-measures ..... 150 2. Life 156 3. Changes during the Progress of the Carboniferous Age 164 4. Mountain-making at the Close of Paleozoic Time . . . 167 Changes in Paleozoic Life at the Close of the Era . 173 III. Mesozoic Time 174 Age of Reptiles 174 1. Rocks . • 175 2. Life 180 3. Mountain-making in Mesozoic Time .... TABLE OF CONTENTS.

XI IV. Cenozoic Time 194 1. The Tertiary, or Age of Mammals 194 1. Rocks 195 2. Life 200 3. Mountain-making 206 4. Climate 209 2. Quaternary Age, or Era of Man 209 1. Glacial Period 211 2. Champlain Period 218 3. Recent Period 221 4. Life of the Quaternary ...... 224 5. Geological Work still going Forward . . . .235 V. Observations on Geological History 237 1. Length of Geological Time 237 2. Progress in Features ....... 240 3. The System of Nature of the Earth had a beginning and will have an end ......... 241 4. Progress in Life 242 INDEX ILLUSTRATIONS. The original sources of the larger part of the illustrations will be found stated in the author's Manual of Geology. Of the few not in the Mannal, Fig. 11 is from a photograph taken by the artist of Powell's Expedition ; Figs. 13 to 16, from the author's "Corals and Coral Islands"; Fig. 33, from H. J. Clarke's " Mind in Nature " ; and Fig. 46, from an electro type kindly furnished by the publishers of the "American Naturalist," Salem.

GEOLOGY. THE word Geology is from two Greek words signifying the story of the earth. As used in science, it means an ac count of the rocks which lie beneath the surface and stand out in its ledges and mountains, and of the loose sands and soil which cover them ; and also an account of what the rocks are able to tell about the world's early history. By a careful study of the nature and positions of rocks, and the markings or relics they contain, it has been discovered how the rocks them selves were made ; and also how the mountains and the conti nents, with all their variety of surface, were gradually formed. And, further, it has been ascertained not only that the earth had plants and animals long before Man appeared, but what were the kinds that existed in succession through the long ages.

The subjects, therefore, of which geology treats are : I. The Kinds of Rocks. II. The ways in which the rocks, valleys, mountains, and continents were made, — or Causes in Geology, and their Effects. i A GEOLOGY. III. The events during the successive periods in the earth's history; that is, what making of rocks was going on in eacli period, what making of mountains and valleys, and what species were living in the waters and over the land in each, and how the world of the past differs from the world as it now is, — all of which subjects, and others related, are treated under the general head of Historical Geology.

Part I Rocks, Or What The Earth Is Made Op

Rocks consist of minerals ; and the ores and gems they con tain are minerals. Any mineral that yields a metal profitably is called an ore. The following are the characters of some of the kinds that are of most importance in geology. I. — Minerals. I . Consisting of Silica. Quartz. — Quartz is the most common of the materials of rocks. It is well fitted for this first place ; for (1) it is one of the hardest of minerals, the point of a knife-blade or edge of a file making no impression on it; (2) it does not melt in the hottest fire ; and (3) it is not dissolved by water, or cor roded by either of the common acids. Its durability is its great quality. With a piece of quartz it is easy to write one's name on glass. Another quality of it, distinguishing it from many minerals it resembles, is that it breaks as easily in one direction as another.

HOCKS, OR WHAT THE EARTH IS MADE OF. It is of various colors and kinds. Flint and hornstone are dark-colored massive quartz. The smooth-surfaced stones of a pebble-bank, whether white, brown, yellow, or black, if uni form (not speckled) in color, are almost all quartz. Moun tains thousands of feet high are sometimes made of quartz rocks. The sands of a sea-shore are mostly quartz, because the grinding of particle against particle which goes on under the heavy dash or swift flow of the waters wears out all other materials, and leaves only the hard quartz particles behind. Quartz is often found in crystals. The figure annexed shows the form of one of them. It is a regular G-sided prism (i i i), with a 6-sided pyramid at each end; and it is often as transparent as glass. Frequently the crystals are attached by one end in great numbers to a surface of rock, so that this surface is brillianl with [ittli Fig. l. ouirtz. pyramids of quartz set crowdedly over it, or with prvamids raised on prisms.

The inclination of the face of the prism to the adjoining face of the pyramid is always the same (141° 47'), wherever the quartz crystal may come from. These glassy crystals are wholly natural productions, having their forms perfect and lustre brilliant when first taken from the rocks. While some quartz crystals are clear and colorless, others have a purple color, and these are the amethyst of jewelry. Others have a light-yellow color, looking like topaz, and are called false topaz; and others a clear smoky-brown color, and these are the cairngorm stone of Scotland. MINERALS CONSTITUTING ROCKS. 5 Still other kinds of quartz are the agates, in which the color is arranged in thin bands or layers of different shades of color, as white, smoky-brown, red, etc. The material of quartz is called in chemistry silica, from the Latin word silex, meaning flint.

Quartz, while so enduring, when pulverized and heated fuses easily with soda, potash, lime, magnesia, or oxyd of iron, and forms a kind of glass ; and ordinary glass is made by melting together quartz sand and soda. Again, hot waters containing soda or potash in solution will dissolve silica, and on cooling deposit it again. The waters of hot springs usually contain silica, which they have taken, along with soda or potash, from some rock with which they have been in contact. Through deposits from such solutions (1) agates have been made; (2) fissures in rocks have been filled with quartz, and the fractures thus mended; and (3) the sands of sand-beds and gravel of gravel-beds have often been cemented into the hardest of rocks. Opal is also silica, but it differs from quartz in being softer, of less specific gravity, and never crystallized ; and in the precious opal it has a beautiful play of colors arising from internal reflections.

The silica of diatoms and of some de posits made by geysers is in the state of opal. 2. Silicates. Silica, while existing in rocks abundantly as quartz, also makes, on an average, a third of all their other minerals, limestones excepted ; that is, it exists combined with other substances, making various common minerals. These minerals containing silica are called silicates. L Feldspar. — The most universal of these silicates are the kinds called feldspar. Besides silica, a feldspar contains the elements of alumina, and of potash, soda, or lime. Corun dum is nothing but alumina ; and the beautiful gem sapphire is only a clear blue variety of it; and the hard emery used for grinding and polishing, and often in little emery bags for sharpening needles, is the same. It is the hardest of all stones excepting the diamond, and hence it is a good companion for quartz or silica in rock-making.

The two, silica and alumina, in combination together make minerals that are harder and no less infusible than quartz ; but when combined also with potash, soda, lime, or iron, the minerals it forms melt more or less easily. Feldspar has usually a white or flesh-red color, and some times might be mistaken for quartz. But (1) it is not quite so hard as quartz, though too hard to be scratched with a knife; and, besides, (2) it melts when highly heated; (3) it breaks in one direction with a bright even surface, brilliant in the sunshine, and also in another direction at right angles or nearly so to the former, but less easily, — a kind of fracture called, in mineralogy, cleavage. While quartz has no cleavage, feldspar has cleavage in two directions transverse to one another. MINERALS CONSTITUTING ROCKS. 7 Common feldspar (called orthoclase in mineralogy) is a potash-feldspar, it containing the elements of potash along with those of alumina and silica ; another is a soda-feldspar (albite) ; others are soda-and-lime feldspars, and one of these, called labradorite, is a constituent of many igneous rocks ; and another is a lime-feldspar. 2.

Mica. — Mica (often wrongly called isinglass) splits very easily into leaves thinner than the thinnest paper, which are tough and elastic, and frequently transparent. It does not melt easily, but fuses on the thin edges with high heat. It is the transparent material commonly used in the doors of stoves. Some mica is white, or gray; it is oftener brownish, and very frequently black. Like feldspar, it contains the elements of silica and alumina; the most common light-colored kind has, besides these constituents, potash ; the black kind contains magnesia and iron. 3. Hornblende. — Black hornblende, when occurring in rocks, often looks much like mica, showing lustrous cleavage surfaces ; but it is a brittle mineral, and hence cannot, like mica, be split into thin, flexible leaves or scales with the point of a knife.

It makes very tough rocks, and hence the first part of the name, horn ; the rocks are heavy and some times look like an ore of iron, and hence the second part, blende, a German word meaning blind or deceitful. It is a silicate, that is, it contains silica, but with it there are iron, magnesia, and lime. There are other kinds of hornblende, but they need not be mentioned here. Augite. — Augite is black or dark-green pyroxene, having the same composition as hornblende, and differing only in the shape of its crystals. It is named from a Greek word signifying lustre, because its crystals are often bright, though not more so than those of hornblende. Two of the crystals of hornblende are represented in Figs. 2, 3, and one of those of augite in Fig. 4. The angle of Figs. 2 - 5. Minerals. Figs. 2, 3.

Hornblende ; 4, Augite ; 5, Gamet in mica schist the prism of augite (or that between / and / in Fig. 4) is about 87° ; while the angle of the prism of hornblende (be tween / and / in Fig. 2) is 124£°; it is owing to this differ ence mainly that hornblende and augite have distinct names. 5. Garnet. — Usually in dark-red crystals, but often also black, and occurring imbedded in mica schist and other rocks ; as represented in Fig. 5, contains silica, alumina, iron, and lime. When transparent it is used as a gem. 3. Carbon and Carbonates Carbon is familiarly known, though in a state not quite pure, as common charcoal. The diamond is crystallized car- MINERALS CONSTITUTING RUCKS. 9 bon, and can be burnt like charcoal, though not without in tense heat. Graphite (or black lead, as it is often badly named, since it contains no lead) is also carbon ; it is the material of lead-pencils.

Carbon combined with oxygen in certain proportions forms carbonic acid, an ingredient of the atmosphere, it constituting 4 parts by volume of 10,000 parts of air; it is the gas that escapes from effervescent waters like soda-water. Its com pounds are called carbonates. L Calcite. — Calcite occurs in crystals that break easily in three directions, affording forms with rhombic faces, like Fig. 6; the angles between the faces are 105° 5' and 74° 55'. A very common form is called dog-tooth spar ; the shape is shown in Fig. 7. Another kind is a 6-sided prism with a low pyramid at either end (Fig. 8). Cal cite is easily scratched with the point of a Figs. 6 - 8. Calcite. knife. In a rock form, it is limestone. When calcite or limestone is burnt, carbonic acid escapes as a gas, and lime (called quicklime, the material that slacks in water and is used for making mortar) is left.

Calcite is carbonate of lime. When a grain of calcite is put into di lute hydrochloric (muriatic) acid, carbonic acid gas is given off freely, producing a brisk effervescence, and the calcite be comes wholly dissolved if it is pure. By means of (1) its effervescence with acid, (2) its low degree of hardness, (3) l* its infusibility in the hottest fire, and its burning to quicklime instead, calcite or limestone is easily distinguished from feldspar and other minerals. The cleavages in calcite also separate it from feldspar; for the number of directions is three, and the angle between them is about 105° instead of about 90°. 2. Magnesian Limestone, or Dolomite. — Limestone sometimes contains magnesia in place of part of the lime, and it is then called, in mineralogy, dolomite, after Dolomieu, a French geologist of the last century.

Dolomite, or magnesian limestone, does not effervesce freely unless the acid is heated, and in this respect it differs from calcite. In aspect, calcite and dolomite are closely alike. 4. Ores. The following are a few of the common ores. L Pyrite. — Pyrite has nearly the color and lustre of brass. It is so hard that it will strike fire with steel (whence its name, from the Greek for fire) , and in this it differs from a yellow ore of copper, called chalcopyrite or copper pyrites, which it much resembles. It is very often in cubes, Fig. 9. J ' like Fig. 9. It consists of sulphur and iron, nearly 48 parts by weight in 100 being iron. Both of these elements have a strong affinity for oxygen; and consequently pyrite often changes to vitriol, or else forms the oxyd of iron called limonite, MINERALS CONSTITUTING ROCKS. 11 It is of no use as an ore of iron, because of the difficulty of separating the sulphur; but it is often employed for the making . of vitriol (sulphate of iron) .

It is the most gener ally distributed of all metallic minerals, occurring in particles through most rocks, crystalline as well as uncrystalline. Ow ing to the tendency to alteration just mentioned, it has caused the destruction or disintegration of rocks over the earth's sur face to a greater extent than any other agency. 2. Magnetite, or Magnetic Iron Ore. — ■ An iron-black ore of iron, having a black powder. It is attractable by the mag net. It is common in Northern New York, Orange County, New York, Sussex County, New Jersey, and many other regions. It consists of oxygen and iron in the proportion of 4 atoms of the former to 3 of the latter, and contains 72 parts of iron in 100. 3. Hematite, or Specular Iron Ore. — A steel-gray ore of iron, but often also bright red, the powder being red.

Red ochre is an earthy hematite. It is not attracted by a mag net. Like magnetite, it occurs in great beds in Northern New York, in the Marquette region, near Lake Superior, in Michigan, and many other places. It consists of oxygen and iron in the proportion of 3 atoms of the former to 2 of the latter, and contains, when pure, 70 parts by weight of iron in 100. All rocks of a reddish or red color owe the color to this oxyd of iron. Hematite and magnetite occur, with small exceptions, in beds instead of veins. When the beds are vertical or nearly so they look like veins. 4. Limonite. — A brown, brownish-yellow, or black ore of iron, affording a brownish-yellow powder, sometimes called brown hematite. Yellow ochre is impure or earthy limonite. It differs in composition from hematite only in containing wa ter; and if heated the water is driven off, and it becomes red, or hematite.

It contains, when pure, about 60 per cent of iron. It is a result of the decomposition of other iron ores, and forms great beds in some regions, as near Salisbury in Connecticut, and Richmond in Massachusetts. It is often found in bogs, and is then called bog-iron ore. Limonite is often disseminated through clays, giving them a yellowish or brownish color ; and such clays when heated turn red, because they lose the water which makes limonite to differ from hematite. This is the reason that bricks are usually red. Clay for making white pottery must contain no iron. 5. Siderite, or Spathic Iron Ore. — A gray to brown ore, without metallic lustre, consisting of oxyd of iron and carbonic acid. When pure about 48 parts in 100 are iron. It occurs crystallized, and also in impure massive nodular forms.

The iron ore of many coal regions is this massive nodular variety. It is a heavy mineral (the specific gravity 3.5 or above), and by this quality it may be distinguished from other grayish or brownish stones. In heated hydrochloric acid it KINDS OF ROCKS. 13 effervesces, owing to the escape of carbonic acid. This ore, like limonite, is sometimes present sparingly in clays. 6. Chalcopyrite, or Yellow Copper Ore. — A brass-colored mineral consisting of sulphur, iron, and copper, about a third of which is copper. It is scratched easily with a knife, and affords a dark-green powder, and thus differs from pyrite, which it resembles. It occurs for the most part in veins with other ores. 7. Galenite, or Lead Ore. — A lead-gray ore, brittle and easily pulverized, and affording a lead-gray powder. It often cleaves into cubic or rectangular forms.

It is the common lead ore. It often contains a little silver, and is sometimes worked as a silver ore. It occurs in cavities in limestones, as in Northern Illinois, Wisconsin, and Missouri, and in Der byshire, England ; and is often found also in veins with other ores. II. — Kinds of Rocks The following are the characters of some of the common kinds of rocks. 1. Limestone; Magnesian Limestone. — These rocks are partly described on pages 9 and 10. They are of dull shades of colors, from white through gray, yellow, red, and brown to black, and of all degrees of texture, from that of flint to a coarse granular texture. The test by acids, by heat, and by a use of the point of a knife in trial of the hardness, are the means of distinguishing limestones from other rocks. Chalk is limestone. Ordinary marble is limestone, and some times the magnesian kind.

The different kinds of limestone are called calcareous rocks, from the Latin calx, meaning lime. 2. Sandstone. — Sandstone is a rock made of sand. The sand may be quartz, like the sand of most sea-shores, or pulverized granite or other rock ; when gathered into beds and consolidated, it makes sandstone. Sandstones are the most common of rocks. They have various dull colors, from white through gray, yellow, and brown to brownish-red and red. 3. Conglomerate. — A conglomerate or pudding-stone is a consolidated gravel-bed, — gravel being sand mixed with peb bles or small stones. The stones are sometimes large, even a foot in diameter. They are often of quartz, sometimes of other hard rocks, and occasionally of limestone. 4. Shale. — Shale is a fine mud or clay consolidated into a rock having a slaty fracture, but less evenly slaty and less firm than true slate.

The colors vary, like the colors of mud or clay, from gray and yellowish shades through red and brown to black. Black is a common color, because the plants and animals that live and die in the mud or over it contain carbon, the chief element of coal, and contribute por tions of carbonaceous substances to the mud. Such black shales, when burnt, usually become white or nearly so, because the vegetable or animal material is then burnt out. For the same reason black limestones afford white quicklime. KINDS OF ROCKS. 15 The loose earthy material of the world, in and out of the water, is mostly either sand, gravel, mud, or clay ; and thus it has been through all ages. Sand is finely pulverized rock. Mud is the same, for the most part; but it may contain rock that is decomposed as well as pulverized.

Clay is a fine kind of mud; it is mainly either pulverized feldspar along with quartz in fine grains, or else decomposed feldspar with more or less quartz. It comes from the pulverizing of granite, gneiss, and other rocks containing feldspar, or from their decomposition. Clay often contains iron; and when burnt to make brick it then becomes red. Gravel is mixed sand and pebbles. The consolidation of sand makes sandstones; of pebble beds, conglomerates ; of fine mud or clay, shale. 5. Argillaceous Sandstone. — When sands are clayey, the beds make, when consolidated, a clayey, that is, argillaceous, sandstone [argilla, in Latin, meaning clay). Such sandstones usually break into thin slabs, in which case they are said to be laminated sandstones; and, if of sufficient hardness, they make good flagging-stone for sidewalks.

The common flag ging-stone used in New York and adjoining States is an argillaceous sandstone. 6. Slate. — Slate, or argillyte, differs from shale in breaking much more evenly, and being much firmer. The slates used for roofing are examples. 7. Granite. — Granite is one of the crystalline rocks, its ingredients being, not worn grains like those of a sandstone or conglomerate, but crystalline grains, — all having been rendered crystalline together by a process in which heat was concerned (p. 26). It consists of grains of three minerals, quartz, feldspar, mica, mixed promiscuously together. The quartz grains are usually grayish or smoky in color (com monly of a darker tint than the feldspar), and have no cleav age. The grains of feldspar have cleavage, and therefore show smooth, sparkling surfaces when a fragment of granite is exposed to the sun, and their color is usually white or flesh-red.

The mica is much softer than the feldspar, and with a point of a knife-blade its grains may be divided into thin, flexible scales; its colors are white, brownish, or black. 8. Gneiss. — Gneiss has the same constituents as granite ; but these constituents are arranged more or less in planes, and, owing to the mica, the rock splits into thick layers, and on a cross fracture appears banded. On account of its split ting into layers gneiss is said to be a schistose rock (this term being derived from a Greek word meaning to divide, and pronounced as if spelt shistose). This schistose structure is the only one distinguishing it from granite. It is somewhat like the laminated structure. 9. Mica schist. — Mica schist has the same constituents as granite and gneiss, but the quartz and mica are much the most abundant, and especially the mica; and on account of the large proportion of mica, mica schist divides into thin KINDS OF ROCKS. 17 layers.

It glistens in the sunshine, owing to the scales of mica. Sometimes the scales of mica are indistinct, and then it is called mica slate. The crystalline rocks, granite and gneiss, and gneiss and mica schist, pass into one another through indefinite shadings. There are granites that are slightly gneiss-like, and all grades to true gneiss ; and there are all grades from gneiss to mica schist, so that it is sometimes difficult to say whether a rock should be called granite or gneiss, and whether another should be called gneiss or mica schist. Again, mica schist shades off through mica slate into argillyte, or clay slate, as the crystalline texture is less and less apparent. 10. Syenyte. — Some granite-like rocks contain hornblende in place of the mica, and such kinds are called syenyte. The hornblende is grayish-black, greenish-black, or black, and differs from black mica in being brittle, and hence in not affording thin, flexible scales.

This fact indicates the kind of examination to be made to distinguish syenyte from granite. The so-called granite of the Quincy quarries, near Boston, and the red Scotch granite imported for monuments, are syenyte. 1L Syenyte Gneiss ; Hornblende Slate. — Syenyte gneiss differs from ordinary gneiss in containing hornblende instead of mica. Hornblende schist or slate is a black slaty rock con sisting mainly of hornblende. B 12. Trap ; Volcanic Rocks. — Trap is an igneous rock : that is, it has cooled from fusion, like the lavas of a volcano. It came to the surface in a melted state, through an opened fissure, from some deep-seated region of liquid rock. The part filling a fissure is called a dike. It has sometimes flowed from the fissure over the adjoining country. Trap is a dark-colored, heavy rock, more or less crystalline in tex ture.