period during which no air-breathing animals lived; but Zoophytes, Shellfish, and Trilobites filled the seas. As fishes were the only representatives of the vertebrates, the age is called the Reign of Fishes.

II. The Secondary Age, comprising the Carboniferous, Trias, Oolite, (including the Lias,) and the Cretaceous formations. Insects, Reptiles, Birds, and Mammals, make their appearance during this period, but as reptiles preponderate, it is called the Reign of Reptiles.

III. The Tertiary Age, embracing the tertiary formations, in which the animals bear close resemblance to those of the present day, belonging generally to the same families, and very frequently to the same genera. Aquatic animals do not preponderate, as in the former ages, but large terrestrial mammalia abound; hence this age is designated the Reign of Mammals.

IV. The Modern Age, comprising all deposits since the Tertiary; characterized by the introduction of the most perfect animals, with man at their head. Some of these became extinct before the creation of man-as the mastodon. This age is denominated the Reign of Man.

CHAPTER IV.

THE UNSTRATIFIED ROCKS.

153. THE igneous rocks differ in chemical composition, in modes of aggregations and in position. They often pass into each other by insensible gradations. Two characteristic minerals, feldspar and hornblende, enter into the composition of all of them. When they have cooled under slight pressure they are porous in texture, as modern lavas; but when they have consolidated beneath great pressure, they are dense and crystalline, as granite.

GRANITIC ROCKS.

154. The granite rocks are highly crystalline, destitute of all traces of stratification, and occur as the basis upon which all the systems of strata repose; they are also thrust up to the surface in mountain masses, or injected into the strata in tortuous veins. They are associated, for the most part, with the oldest strata: they constitute the axes of principal mountain chains of the globe.

155. The characteristic ingredients of granite are quartz, w feldspar, and mica, mingled in variable proportions. The prevalent color will depend upon the predominating ingredients; white and flesh-colored are common hues. The rock is either coarse-grained, when the distinct crystalline frag-a ments of which it is composed are large; or fine-grained, when they are very small.

This structure is

Fig. 78.

The quartz and feldspar are sometimes so arranged, as to present figures resembling Hebrew or Arabic letters. The rock is then called graphic granite. frequently exhibited by granite veins, which occur in older granite. When granite contains large distinct crystals of feldspar, it is denominated porphyritic. Granite appears, in some instances, as at Arran in Scotland, to have been protruded through the strata after it had consolidated.

Graphic Granite.

156. In Syenite the three essential constituents are quartz, feldspar, and hornblende. It derives its name from Syene in Egypt, where it was formerly extensively quarried. The rock, composed of quartz, feldspar, mica, and hornblende, is called Syenitic Granite. Syenite is of a dark-green, or black color.

157. The term porphyry (porphura-purple) was applied by the ancients to a rock consisting of crystals of Fig. 79. reddish feldspar imbedded in a base, of which feldspar was also an abundant ingredient. The rock was hard, susceptible of polish, and was used for architectural purposes, sarcophagi, &c. The term porphyry is now used to designate a rock of uniform compact base, through which are disseminated distinct crystals of any mineral. Crys

Porphyry.

tals of feldspar occur in basalt and trachyte, constituting

basaltic porphyry, and trachytic porphyry. Instead of feldspar the imbedded crystals are sometimes of hornblende, or olivine.

158. Metaliferous veins, are most numerous in the primary and transition rocks; they rarely occur worth working above the coal. The ore does not usually occupy the whole vein, but is mixed with quartz, granite, porphyry, sulphate of baryta, &c., which constitutes the vein-stone, gangue, or matrix. Metallic veins differ greatly in thickness and extent; the contents of the same vein also vary in its different parts. Metallic veins are most productive when they pass through the junction of the unstratified and stratified rocks: their productiveness is also influenced by their direction; the east and west courses of the tin and copper veins of Cornwall are productive, while the cross courses— north and south veins-are not worth working.

TRAPPEAN ROCKS.

159. Certain crystalline rocks, composed of feldspar, co and augite or hornblende, are called trap rocks, from the Swedish word trappa, a stair, because they frequently occur in tabular masses rising one above another, as terraces or steps. They appear to have been ejected as lava through fissures, and have produced the characteristic effects of heat upon the adjacent rocks. The trap rocks differ in constitution from the granitic, in containing much less silica, more magnesia, lime, alumina, and oxide of iron; and in being more fusible. The trap rocks occur in dikes, in domeshaped masses covering other rocks, and in regular pillars; they are usually compact, but sometimes are full of pores. There are several varieties of trap rocks.

160. Greenstone consists of an intimate mixture of feld

1

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spar and hornblende of a dark green color, and is either compact or crystalline. When the rock is composed of albite and hornblende in grains it is called diorite. The structure of greenstone is sometimes massive, but more frequently tabular.

161. The constituents of Basalt are augite and feldspar. When the former ingredient prevails, as is usually the case, the rock is of a dark green, or black color, which is due to the iron in the augite. Another metal, titanium, is usually an ingredient, and the mineral, olivine, is often found in grains or nodular masses in basalt. When coarse crystals of feldspar are disseminated in the rock, it is called porphyritic basalt. The structure of this rock is usually columnar.

162. Trap rock, containing almond-shaped cavities, is called amygdaloid, (from amygdalum, an almond.) These cavities are sometimes elongated by the flowing of the melted matter into cylinders several inches long. The cavities of the amygdaloid are filled with zeolites, quartz, calcareous spar, &c. A soft and earthy variety of trap, resembling indurated clay, is called wacke or toadstone.

163. Trachyte is composed of feldspar, with a small proportion of hornblende, titaniferous iron and mica. It derives its name from its roughness, or harshness to the touch-trachus in the Greek language signifying rough. It appears frequently in pulverulent masses of pumice, called tufa or tuff in rocks of all ages, invariably indicating the vicinity of erupted igneous rocks. Trachyte usually contains distinct crystals of feldspar, and consequently is porphyritic; it is also sometimes designated trachytic porphyry. A gray variety of basalt, consisting principally of feldspar, produces a clear ringing sound when struck with a hammer, and is called clinkstone.