. Elements of geology : a text-book for colleges and for the general reader. Geology. 202 UNSTRATIFTED OR IGNEOUS ROCKS. axes and peaks of nearly all great mountain-ranges; lb', in vertical or nearly vertical sheets, filling great fissures in stratified or in other igneous rocks; c, in extensive horizontal sheets overlying the stratified country rock, as if outpoured on the surface; c\ lying conformably between strata, as if forced in a melted condition between them, or else. 0^S^yr i~C-~ *â * x ;;;tjMesozoic. |=^z]Cenozoic. Fig. 180.âDiagram showing Mode of Occurrence of Igneous Rocks. outpou
. Elements of geology : a text-book for colleges and for the general reader. Geology. 202 UNSTRATIFTED OR IGNEOUS ROCKS. axes and peaks of nearly all great mountain-ranges; lb', in vertical or nearly vertical sheets, filling great fissures in stratified or in other igneous rocks; c, in extensive horizontal sheets overlying the stratified country rock, as if outpoured on the surface; c\ lying conformably between strata, as if forced in a melted condition between them, or else. 0^S^yr i~C-~ *â * x ;;;tjMesozoic. |=^z]Cenozoic. Fig. 180.âDiagram showing Mode of Occurrence of Igneous Rocks. outpoured on the bed of the sea and afterward covered with sediment; and, d, in tortuous veins connected with the great underlying masses. All these positions are illustrated in Fig. 180. In all these modes of occurrence the observed rock is connected with an underlying mass, of which it is but an extension. Extent oil the Surface.âThe appearance of these rocks on the sur- face is far less extensive than that of the stratified rocks. Certainly not more than one tenth of the land-surface is composed of them. But, beneath, they are supposed to constitute the great mass of the earth. Classification of Igneous Rocks.âIgneous rocks are best classified, not by means of their relative ages, but partly by their mineralogical character and partly by their mode of occurrence. By this method they most naturally fall into two primary groupsâviz., the Plutonic or massive, and the volcanic, or true eruptive rocks. The rocks of the first group occur in great masses; those of the second group injected into fissures or outpoured on the surface. The former are entirely crystalline (holo-crystalline), and usually very coarse-grained (macro- crystalline) ; the latter are usually finer grained (micro-crystalline), or imperfectly crystalline (crypto-crystalline), or partly or even wholly glassy. The former seem to have solidified in, situ (indigenous); the latter have been evidently displaced form th
Size: 2447px × 1021px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
