. Elements of geology. Geology. 272 LYELL'S ELEMENTS OF GEOLOGY. Tests of Relative Age of Volcanic Rocks. tion at D (Fig. 284.), we may perhaps ascertain that the trap h flowed over the fossiHferous bed c, and that, after its consolida- tion, a was deposited upon it, a and c both belonging to the same geological period. But if the stratum a be altered by h at the point of contact, we must then conclude the trap to have been intrusive, or if, in pursuing h for some distance, we find at length that it cuts through the stratum a, and then overlies it. We may, however, be easily deceived in suppos
. Elements of geology. Geology. 272 LYELL'S ELEMENTS OF GEOLOGY. Tests of Relative Age of Volcanic Rocks. tion at D (Fig. 284.), we may perhaps ascertain that the trap h flowed over the fossiHferous bed c, and that, after its consolida- tion, a was deposited upon it, a and c both belonging to the same geological period. But if the stratum a be altered by h at the point of contact, we must then conclude the trap to have been intrusive, or if, in pursuing h for some distance, we find at length that it cuts through the stratum a, and then overlies it. We may, however, be easily deceived in supposing a volcanic rock to be intrusive, when in reality it is contemporaneous, for a sheet of lava, as it spreads over the bottom of the sea, cannot rest every where upon the same stratum, either because these have been denuded, or because, if newly thrown down, they thin out in certain places, thus allowing the lava to cross their edges. Besides, the heavy igneous fluid will often, as it moves along, cut a channel into beds of soft mud and sand. Suppose the sub- marine lava F, to have come in contact in this manner with the strata a, &, c, and that, after its Fig. 285. consolidation, the strata J, e, are thrown down in a nearly horizon- tal position, yet so as to lie uncon- formably to f, the appearance of subsequent intrusion will here be complete, although the trap is in fact contemporaneous. We must, unless we find the strata (Z or e to have been altered at their junction, as if by heat, not therefore hastily infer that the rock f is intrusive. When trap dikes were described in the 8th chapter, they were shown to be more modern than all the strata which they tra- verse. The ninety-fathom dike in the Northumberland coal- field (see section, Fig. 286.) passes through coal-measures which Fig. 286. Magnesian Dike. Section in a quarry at Cullercoats, Northumberland. are much disturbed.* The beds of overlying Magnesian lime- stone are not cut through by the dike, but
Size: 3532px × 707px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorlyellcharlessir17, bookcentury1800, booksubjectgeology
