Geology And Ore Deposits Of The Blewett Mining District (1911) (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: CHAPTER II. GENERAL GEOLOGY. INTRODUCTION. The general geology of the Blewett mining district is but a small fragment of the geology of the eastern portion of the Cascade mountains. Because of this fact, the elucidation of nearly all of the intricate geologic problems involved in this district must be partially sought for in the broader field surrounding this district. A part of this broader field has been studied and areally mapped and published in the Mount Stuart and Sno- qualmie geological folios. Hence all of the formations occurring in the Blewett district have been previously described, and in the following discussion an attempt will be made to represent in as much detail as possible the more salient features of each formation and their relation to the broader field outside, and the problems involved in the deposition of the ore. Many of the formations occurring outside of the district do not occur within it. The oldest formation found in this district consists of a lower series of quartzites, slates, quartz- ite-conglomerates and cherts, overlaid apparently unconforma- bly by an upper series of brecciated lavas and tuffs, together with massive igneous rocks. No fossiliferous evidence has been obtained to determine their age, but from their close resemblance to formations in the north, whose age is known, they are provisionally assigned to the Carboniferous. After their deposition they underwent deformation resulting in their meta- morphism. Accompanying this deformation were great intrusions of peridotite and other associated basic igneous rocks which were later more or less altered to serpentine. After this, Ann. Report, Geological Survey, Canada, New Series, Vol. 7, 1894, pp. 37 B-49-B. erosion became active and deformational movements produced a warping, al...