A Contribution to the Geology of the Northern Black Hills (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1899 Excerpt: ... of meta-diabases and meta-gabbros whose chief components are plagioclase feldspar, hornblende of the variety uralite and accessory ilmenite, calcite, apatite and quartz. Cores of large dikes show uralite masses with augite kernels, furnishing a clue to the original character of the rock. Such cores pass towards the periphery with increasing schistosity, proceeding by insensible gradations into chlorite and hornblende schist. DETAILED DESCRIPTION, i. Quartz-segrite-porphyry. This rock is of an exceedingly interesting character as it corresponds exactly in some cases to rocks in Norway, described by W. C. Broegger, as Grorudite. In their general appearance these rocks are massive and homogeneous. They range in color from a light greenish gray to almost a white in the more coarse grained and porphyritic varieties. The texture varies from almost completely aphanitic, through porphyritic up to one almost granitoid. In some varieties the quartz cannot be observed in the field, but two very interesting occurrences exhibit very large and numerous phenocrysts of quartz, which have led previous observers to class them with the rhyolites. The quartz-aegirite-porphyries will be described under the following types: 1. Lost Camp Creek type. 2. Elk Mountain type. 3. Terry Peak type. 4. Annie Creek type. 5. Sunset Mine type. 6. Bald Mountain type. 1. Lost Camp Creek Type. Megascopic Appearance.--As seen in the field this rock is a light greenish gray and is very fine-grained. The phenocrysts consist of orthoclase, in sparsely scattered crystals, and of very small aegirines which may be seen distributed through the rock, forming here and there a noticeable dark speck or needle-like crystal. The groundmass which constitutes almost the whole body of the rock is a light grayi...