Bulletin - Colorado Geological Survey Volume 3 (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: some distance up the slopes of Mount Bross, forms practically the whole of Bald Hill to the east of Pennsylvania Mountain, and comprises, with the included porphyries, the whole of the area southwest of the London fault. As above shown, igneous rocks are to be found in all the formations of the district, from the Archean up to the Weber grits. In the Archean they are mostly granites and diorites, which cover considerable territory, while the porphyries are confined to narrow dikes. In the sedimentary series the intruded porphyries appear to occur in the lower formations only in comparatively thin sheets or in narrow dikes, but in the Weber formation, and to a less extent in the Carboniferous limestone, these sheets attain at times very great thickness, being not less than one thousand feet thick in the extreme southwest corner of the surveyed area. At the same time, they are inclined to depart from conformable sheets and occur in more irregular masses that cut the sedimentary formations at various angles, and that often cause the bedded rocks to show very abnormal strike and dip. Glociation phenomena are very pronounced over nearly all of the district. In the lower valleys extensive glacial moraines deeply bury the rock formations beneath them. In the upper valleys the moraine material occurs in great abundance, but does not so completely bury the other formations as to altogether hide their identity. For this reason it has been possible to map the underlying rock formation in spite of the covering of morainal drift. There is no very extensive deposit of alluvium in the district covered by this survey. It is practically confined to a few hundred feet along the bottom of two or three of the streams. The easterly dip of the sedimentary formations already mentioned, while...