. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 660 VOLCANIC ROCKS OF [Ch. XXX ing (fig. 712), the overlying clay has been greatly altered and hard- ened by the igneous rock, and occasionally contorted in the most extraordinary manner; yet the lamination has not been obliterated, but, on the contrary, rendered much more conspicuous, by the indu- rating process. In the foregoing woodcut (fig. 713) I have represented a portion of the altered rock, a few feet square, where the alternating thin laminse of sand and cla
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 660 VOLCANIC ROCKS OF [Ch. XXX ing (fig. 712), the overlying clay has been greatly altered and hard- ened by the igneous rock, and occasionally contorted in the most extraordinary manner; yet the lamination has not been obliterated, but, on the contrary, rendered much more conspicuous, by the indu- rating process. In the foregoing woodcut (fig. 713) I have represented a portion of the altered rock, a few feet square, where the alternating thin laminse of sand and clay have put on the appearance which we often observe in some of the most contorted of the metamorphic schists. A great fissure, running from east to west, nearly divides this larger island into two parts, and lays open its internal structure. In the section thus exhibited, a dike of lava is seen, first cutting through an older mass of lava, and then penetrating the superincumbent ter- tiary strata. In one place the lava ramifies and terminates in thin veins, from a few feet to a few inches in thickness (see fig. 714). Fig. b a 5 c a & Clay. Lava. Clay. Altered. Lava, Clay, &c. Newer Pliocene strata invaded by lava, Isle of Cyclops (horizontal section). a. Lava. &. Laminated clay and sand. c. The same altered. The arenaceous laminas are much hardened at the point of contact, and the clays are converted into siliceous schist. In this island the altered rocks assume a honeycomb structure on their weathered surface, singularly contrasted with the smooth and even outline which the same beds present in their usual soft and yielding state. The pores of the lava are sometimes coated, or entirely filled, with carbonate of lime, and with a zeolite resembling analcime, which has been called cyclopite. The latter mineral has also been found in small fissures traversing the altered marl, showing that the same cause which introduced the minerals into the cavities of the lava
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
