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CHAPTER IX.

ON UNCONFORMABLE ROCKS OF PORPHYRY, TRAP AND BASALT, AND ON BASALTIC DYKES.

The different Positions of Conformable and Unconformable Massive Rocks described.-Opinions respecting the formation of Unconformable Massive Rocks.-Varieties of Trap Rocks; their Passage by Gradation into each other and into Volcanic Rocks.-Porphyry, Porphyritic Trap, Greenstone, Sienite, Clinkstone, Basalt, Amygdaloid, Wacke, Pitchstone.Passage of Porphyry and various Trap Rocks into each other, and into Sienite and Granite, at Christiania in Norway.Passage of Basalt into scoriaceous Lava and Obsidian.Mountains of Porphyritic Trap and Clinkstone with deep Craters, probably formed by depression.-High Stile, Cumberland.-Cader Idris, Wales.-Basaltic Dykes.-Columnar and Massive Basalt.-Interstratified Basalt.-Strata confusedly broken and enveloped in Basalt.-Organic Remains enveloped in Basalt.-Basalt of Scotland, Ireland, Auvergne, and Iceland. On the formation of Basalt.-Experiments of Mr. Watt.-Theory of Werner.-On the relative age of Trap Rocks.

THE rocks described in the preceding chapters, both of the primary, the transition, and the lower secondary class, generally cover each other in a conformable position; and where they occur together, their arrangement and order of succession may be represented as in Plate 3. fig. 1. where the granite a is covered by gneiss b, and this by mica-slate c. Next succeed the slate rocks dd, with imbedded or subordinate beds of flinty slate, or

limestone (1). Then follow beds (2) of conglomerate, separating the slate from the transition limestone and greywacke (2. d. d). And lastly, the lower secondary strata, comprising the great coal formation. This representation is, however, rather descriptive of a general tendency to such an arrangement, than of its universal occurrence. Frequently different parts of the series will be wanting in different situations. Thus in Cornwall the gneiss and the mica-slate are absent, except in two localities. At the Malvern Hills, transition limestone rises almost close to the rocks of granite: and the coal formation of St. Etienne in France is only separated from the granite, on which it reposes, by a thick bed of conglomerate, containing rounded stones of vast size, belonging to rocks of the primary class. Here all the intervening formations, b. c. d. and e. Plate 3. fig. 1. are wanting. Whether they were originally deposited, and have been subsequently removed, by the agency of some unknown cause, before the deposition of the coal strata, is an inquiry that can only be answered by conjectures and references to analogies: the subject will be hereafter adverted to. It is sufficient to our present purpose to remark, that where the different formations that are wanting in one place occur in another, they generally cover each other conformably in the order above described.

The upper secondary and the tertiary strata, that succeed the different classes of rocks which occur conformably, appear to have been deposited not only at a later period, but under different circumstances; for their position does not conform to that of the lower

rocks, but they cover the outcrop or basset of the lower beds, as represented Plate 1. fig. 3; they are unconformable stratified rocks.

Before we proceed to describe the upper secondary strata, it will be necessary to bestow particular attention on a class of rocks, that has greatly perplexed the speculations of geologists: they are the unconformable rocks of porphyry, trap, and basalt, which form the subject of the present chapter. These rocks, though they sometimes occur imbedded in conformable rocks, more frequently cover them in an unconformable position, composing thick unstratified beds, and often mountain masses of vast size, that have not unfrequently a columnar structure. Their position is represented Plate 3. fig. 2. It is obvious that these unconformable rocks were formed and deposited, at a subsequent period to that in which the lower rocks were consolidated, and their beds had acquired their present inclined positions. As the unconformable massive or unstratified rocks, are many, if not all of them, allied to rocks whose igneous origin is now undisputed, we might have little difficulty in admitting that they had been poured over the surface of the conformable rocks in a state of fusion, like streams of lava from recent volcanoes; with this difference, that they were not erupted from one opening or crater, but from fissures of great width and many miles or leagues in extent, and that they were formed under the ocean. I say we might have little difficulty in admitting this, particularly as such rents or fissures, filled with similar matter to that of the overlying unconformable masses, are often discovered in their

vicinity: but there are other appearances which seem opposed to the igneous origin of these rocks. In the first place, similar rocks to the overlying formations, are often imbedded and intermixed with conformable rocks in a conformable position: and secondly, some overlying formations, appear to pass by gradation into rocks of the primary class. Now if we admit the unconformable masses to be of igneous origin, we can scarcely refuse the same origin to the imbedded rocks; and thus we extend the domain of Pluto over a larger portion of the crust of the globe, than many geologists will allow. It must also be granted, that many of the imbedded rocks of porphyry and porphyritic trap, which alternate or are intermixed with slate rocks, cannot be supposed to have ever been erupted like lava; but they may have been softened by subterranean heat in situ, and have taken the porphyritic texture, and the columnar structure, during their slow refrigeration. If we sufficiently keep in view that the crust of the globe with which we are acquainted, does not exceed, in comparative thickness, that of a wafer to an artificial globe three feet in diameter; and that a very large portion of the globe is now or has in ancient times been rent and pierced through by active volcanoes, and that these volcanoes are not the seat of subterranean fire, but merely its chimneys, we shall have no difficulty in admitting, that extensive parts of the crust of the globe may have been softened by internal heat, and the more fusible beds partly crystallized in situ, under the pressure of the ocean.

Trap rocks sometimes occur between thick beds

of marine limestone; but in such instances, we may without much difficulty admit that these trap rocks have been formed by submarine volcanoes, which have poured beds of lava over the limestone; another bed of limestone may have been subsequently formed over the lava, and this limestone may also have been covered by the lava of a later eruption. In this manner the alternation of beds of basalt, or basaltic amygdaloid with limestone in Derbyshire, may admit of a probable explanation.

With respect to the overlying formations which pass by gradation into primary rocks, as some porphyries allied to volcanic rocks pass into granite,this fact, so far from proving that the porphyry was not of igneous origin, would tend to confirm the hypothesis which attributes an igneous formation to granite itself. It is granted by the best ob

* However highly and justly distinguished many of the natural philosophers in France may be, it cannot be denied that they adhere more closely to theories once formed, and have a greater dread of thinking for themselves, than the philosophers of other countries. In confirmation of this, I shall translate an extract from M. Bonnard's Aperçu Geognostique des Terrains. It is truly amusing to see the alarm which he evinces, lest he should be compelled by stubborn facts to relinquish his cherished theories." Another species of difficulty should prevent every prudent man (esprit sage) from attempting to explain the formation of these rocks of trachyte by any hypothesis founded on volcanic action; namely, the alarming extent of the consequences which may follow such an explication, relative to other rock formations, hitherto regarded as having a very different origin." With great respect for M. Bonnard, I would say, Let every esprit sage yield to the evidence which Nature presents, and leave consequences and theories to take care of themselves.