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Show 366 STRATIFIE'D RO<.:KS [Ch.XXVI. Crag and other formations fk.. This disposition of the layers No. 89. Lamination of clay-slate, Montagne de Seguinat, near Gavarnie, in tl1e Pyrenees. is illustrated in the accompanying diagram, in which I have represented carefully the stratification of a coarse argil~aceous schist, which I examined in the Pyrenees, part of which approaches in character to a green and blue roofing slate, while part is extremely quartzose, the whole mass passing downwards into micaceous schist. The vertical section here exhibited is about three feet in height, and the layers are sometimes so thin that fifty may be counted in the thickness of an inch. Some of them consist of pure quartz. The stratification now alluded to must not be confounded with that fissile texture sometimes observed in the older rocks, by virtue of which they divide in a direction different both from the general planes of stratification and from the planes of those transverse layers of which a single stratum may be made up. . . Another striking point of analogy between the stratificatiOn f the crystalline formations and that of the secondary and 0 • tertiary periods is the alternation in each of beds varymg greatly in composition, colour, and thickness. We observe, for instance, gneiss alternating with layers of black hornbl~ndeschist, or with granular quartz or limestone, and t~e mt~rchange of these different strata may be .repeat~d for an mdefin~te number of times. In like manner, mica-schist alternates w1th chlorite-schist, and with granular limestone in thin layers. As we observe in the secondary and tertiary formations • See above, p. 173. Ch.XXVI.] CALLED ' PRIMARY.' 367 strata of pure siliceous sand alternating with micaceous sand and with layers of clay, so in the 'primary' we have beds of pure quartz rock alternating with mica-schist and clay-slate. As in the secondary and tertiary series we meet with limestone alternating again and again with micaceous or argillaceous sand, so we find in the 'pdmary' gneiss and mica-schist alternatinO' with pure and impure granular limestones. 0 Passage of gneiss into granite-If, then, reasoning from the principle that like effects have like causes, we attribute the stratification of gneiss, mica-schist, and other associated rocks , to sedimentary deposition from a fluid, we encounter this diffi-culty, that there is often a transition from gneiss, one of the stratified series, into granite, which, as we have shown, is of igneous origin. Gneiss is composed of the same ingredients as granite, and its texture is equally crystalline. It sometimes occurs in thick beds, and in these the rock is often quite undistinguishable, in hand specimens, from granite; yet the lines of stratification are still evident. These lines imply deposition from water, while the passage into granite would lead us to infer an igneous origin. In what manner can we reconcile these apparently conflicting views P The Hutton ian hypothesis offers, we think, the only satisfactory solution of this problem. According to that theory, the materials of gneiss were originally deposited from water in the usual form of aqueous strata, but these strata were subsequently altered by their proximity to granite, and to other plutonic masses in a state of fusion, until they assumed a granitiform texture. The reader will be prepared, by what we have said of granite, to conclude, that when voluminous masses of melted rock have been for ages in an incandescent state, in contact with sedimentary deposits, they must produce some alteration in their texture, and this alteration may admit of every intermediate gradation between that resulting from perfect fusion, and the slightest modification which heat can produce. The geologist has been conducted, step by step, to this |
