It is equally certain that the commencement of their activity was at a late period in the history of the earth. Lava currents are frequently found in France resting upon the early tertiary strata, but no lava current is found below them. The later tertiary strata contain pebbles of volcanic rocks, showing that lavas had been previously ejected, but none are found in the older strata of this formation. We must, therefore, conclude that these volcanic tracts assumed their volcanic character at some intermediate point in the tertiary period.
When we find that their activity commenced at so late a period and closed so long ago, we might be led to suppose that it was of very short duration. But a great number of facts, in the present condition of the country, require that we should assign to them a very prolonged activity. A single instance will be sufficient to show the nature of the evidence upon which this conclusion rests. The heavy line ([Fig. 6]) represents the present form of one of the valleys. A bed of lava forms the highest point of land represented, and a second bed is found in an intermediate part of the slope. The position of the upper bed must have been a valley, when the lava flowed there. We may represent this valley by the line a b c. The slow operation of natural denuding causes at length excavated the valley d e h, when another lava current flowed through it, covering its bed of pebbles, as before. The same denuding causes have at length produced the present valley, f g h. These remnants of lava-currents, as they have formed a very imperishable rock, have protected the subjacent strata from erosion, and furnish evidence of the position of the valley at different periods. When we consider with what extreme slowness denuding causes produce changes on the surface, and what extensive changes they have here nevertheless effected in the interval between the production of the different lava currents, we are compelled to feel that that interval was a very prolonged one. Yet this period, however long it may have been, was evidently less than the period of activity of these volcanoes.
3. Volcanic rocks of an earlier date are also found, sometimes as distinct lavas, though generally as volcanic grits. They occur interstratified with the cretaceous rocks, and with every other formation of the fossiliferous series, showing that, from the earliest times, these rocks have been accumulating as they now are.
The trappean rocks may, in a general classification, be considered as volcanic. It will be shown, hereafter, that they are the lavas of submarine volcanoes. They do not, however, occur in the form of lava currents, but in great tabular masses, generally between stratified rocks, or in the form of dikes. They are also entirely unconnected with cones or craters.
The trappean rocks occur more or less abundantly in all countries. One of the most noted localities of this rock is a region embracing the north of Ireland, and several of the islands on the western coast of Scotland. It contains the celebrated Giant’s Causeway, which consists of a mass of columnar trap; also Fingal’s Cave, which is produced by a portion of the trap being columnar, and thus disintegrating more rapidly than the rest, by the action of the waves. An immense mass of greenstone trap, which has generally been considered as a vast dike, though often a mile in thickness, is found extending from New Haven to Northampton, on the west side of the Connecticut river. It then crosses to the east side, and continues in a northerly direction to the Massachusetts line. Under different names, it constitutes a nearly continuous and precipitous mountain range for about one hundred miles. Dr. Hitchcock supposes this greenstone range to be, not an injected dike, but a tabular mass of ancient lava, which was spread out on the bed of the ocean during the period of the deposition of the Connecticut river sandstone. It was subsequently covered with a deposit of strata of great thickness, and then by subterranean forces thrown into its present inclined position.
There is a mass of basaltic rock in the valley of the Columbia river, in the Oregon Territory, which extends without interruption for a distance of four hundred miles. Its breadth and thickness is not known, but in some places the river has cut a channel in this rock to a depth of four hundred feet. Its age has not been determined, and it will, perhaps, be found to be a tertiary or modern production.
SECTION IV.—THE NON-FOSSILIFEROUS STRATIFIED (OR METAMORPHIC) ROCKS.
1. Gneiss is the most abundant rock in this class, and is generally found reposing on granite. Its stratification is sometimes very distinct, but it is often so imperfect that it can scarcely be recognized. This is more frequently the case in the vicinity of granite on which it rests, and into which it insensibly passes. A large part of the material used for building purposes, under the name of granite, is obscurely marked gneiss. In all primary countries it is an abundant rock, occupying extensive districts, and sometimes forming mountain masses.