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Bulletin of the Museum of Comparative Zoology at Harvard College, 1888

Excerpt from Bulletin of the Museum of Comparative Zoology at Harvard College, 1888 In the region north of Boston occurs a most interesting series of massive rocks, which break through the slates and sandstones, and include granite, quartz-porphyry, quartzless-porphyry, elaeolite-syenite, diorite, porphyrite, diabase, augite-porphyrite, and gabbro. They have been studied in greater or less detail by many observers, prominent among whom are W. O. Crosby, M. E. Wadsworth, and J. S. Diller. The rocks which have afforded material for the present study belong to a single dike, and may be seen in a series of exposures in Medford and Somerville. They are coarsely crystalline rocks, and have bourne the names "syenite" and "diorite." They have in general been carefully distinguished from similar rocks of finer texture known as "greenstones," which were shown by Wadsworth in 1877 to have about the same composition as the coarser rock, and were considered by him as identical with it. The finely crystalline rock seems to be more widely distributed than the coarse variety. In the present paper it has been studied only at a few localities, where there was some promise of deciphering its relations with the "diorite.' The age of these rocks has not been accurately determined though they have generally been considered post-triassic on account of their lithological resemblance to the diabase of the Connecticut Valley. The slates through which they have broken are probably identical with the Lower Cambrian argillite of Braintree. Diller has furnished evidence to show that the finely crystalline diabase ("greenstone") is the youngest of the eruptives of this region, its dikes cutting those of the other rocks. Many mistakes have been made in determining the composition of both the so-called "diorite" and "greenstone." About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at 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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