Excerpt from Memoirs of the National Academy of Science, 1927, Vol. 19 From the various considerations already adduced it Will be quite clear that no general philosophy of petrogenesis should be based upon a study of a portion of the 1gneous recks alone, however complete-it might be' in regard to geological, petrographical, and physico-che'mical circumstances; and therefore, since no study of the intermediate and acid types of normal and alkaline 1gneous rocks has been made by the writer comparable in detail with those upon basic igneous rocks here given, it must be recognized that the generalizations which follow are derived perhaps too exclusively from the' study of basic rocks. If, however, they are found to be an adequate statement of the modes of occurrence of such rocks, they may then be combined with the results of s1m11ar studies upon the other types of rocks into a general petrogenetic theory. While no invariable rules for mode of occurrence of basic intrusive rocks seem yet dis coverable, there emerge from the studies which follow the conception of several modes of occur 'rence, so frequently represented that they' seem to result from fundamental principles of petrogenesis, while the exceptions to these are especially worthy of study,.s1nce in all probability they will throw most light upon the nature of the controlling conditions and the relationships to one another of apparently distinct modes of occurrence. 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 s