The Journal of Geology, Vol. 23

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Excerpt from The Journal of Geology, Vol. 23: A Semi-Quarterly Magazine of Geology and Related Sciences, January-December 1915Inclusions are not infrequent in all types of both extrusive and intrusive rocks. Those in extrusive rocks have been specially treated by Lacroix. Intrusive rocks are so extensive and of such varying forms that only dikes and a few related intrusions will be considered here. The xenoliths in the large intrusive bodies have formed the basis for the stoping hypothesis, but. Only part of the inclusions in dikes are of this origin.Dikes acquire inclusions by Shattering blocks Off the walls of a fissure during their ascent through it. These fragments may remain near the place from which they came, or they -may move up or down. In most cases the fragments rise, whether of greater specific gravity than the molten dike - rock or not, because they are forced upward by the magma. In the cases where the fragments Sink, either they are heavier than the magma or. They are carried down.Some dikes have invaded conglomerates, from which they have dissolved the cement and included the bowlders. In other cases the fissures through which the magma came may have been open to the surface, SO that stream gravels fell in and Were caught in the ascend ing magma. In rare instances a dike may ascend through a fault breccia and thus acquire its inclusions. These special cases are not separated from the other examples.To Classify dikes according to the direction of movement Of the inclusions is difficult, because in almost every case some inclusions rise and some remain near the place Of origin. A classification is attempted which will at least serve to emphasize the number Of examples in which inclu'sions sink.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.