Excerpt from Rock Minerals: Heir Chemical and Physical Characters and Their Determination in Thin Sections The term Rock Minerals is sufficiently indefinite to permit of con siderable latitude in the choice of minerals to be included by it. Besides those that constitute the mass of any rock there are the less abundant though common kinds, as well as Occasional, exceptional, ones. In any case the mineral may be an original constituent of the rock or one that has been developed in it subsequent to its formation. Rock Minerals, therefore, embrace not only all primary minerals, but all those of secondary origin produced by any manner of alteration of previously existing minerals. Among such secondary minerals are some that are better known as fillings of cavities and veins in rocks than as rock constituents, such as the zeolites. These, how ever, also occur as replacements of primary crystals within the body of rocks, and so constitute rock minerals in a narrower sense. The term becomes somewhat more comprehensive when it is applied to minerals formed in pegmatite veins, which are undoubtedly rocks of intrusive, igneous, origin, for there are many uncommon minerals well developed in such veins that are not known to crystallize within the main mass Of larger rock bodies. However, it is customary to exclude certain classes of veins of minerals, such as ore veins, from the category of rocks, though their mode of formation in some instances is the same as that of many kinds of secondary rock minerals, and their study by petrographical methods in thin sections has been undertaken more or less systematically by several investigators. The inclusion of such vein minerals within the field of the petrographer would place almost the whole range of mineralogy before him and cause a distinction between the petrographer and mineralogist to consist, as it virtually does at present, in the objects and methods of research rather than in the materials investigated. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This 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.

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