150 GEOLOGY AND 'MINERALOGY. 



clay slates, mica schists, seams of white quartz, granite composed largely of 

 feldspar, and we can readily detect the source of the fragmentary masses which 

 form the conglomerates. We also know that while nuclei of certain mountain 

 ranges on the eastern slope are composed of a massive feldspathic granite, a 

 great thickness of more recent or overlying rock, forming the lower and smaller 

 ridges are composed of a kind of ' rotten granite,' which is so full of the hydra- 

 ted oxyd of iron that it readily decomposes on exposure to the atmosphere. We 

 therefore believe that the source of all the sediments composing the Primordial 

 rocks in the West can be traced to the underlying rocks in the vicinity. 



" 4. There are no indications of long continued deep water in the Primordial 

 sea, so far as the West is concerned. If we examine the lower part of the Pots- 

 dam sandstone we find that the physical conditions which ushered in this period 

 were quite violent, as shown by the conglomerate character pf the rock. Pass- 

 ing upward, this conglomerate graduates into a rock composed of granules of 

 quartz and small plates of mica cemented with calcareous matter, and about 

 midway in the formation we have a fine, very ferruginous calcareous sandstone, 

 in thin layers, filled with fossils in a very good state of preservation. The 

 condition of the organic remains, the fineness of the sediment, and the perfect 

 horizontality of the laminae of deposition indicate a short period at least, of 

 quiet water. As we continue upward the rocks begin to show the shifting 

 nature of the currents, shallow water, and perhaps a proximity to land, by 

 obliqne laminge of deposit, ripple markings and fucoidal remains. The upper 

 portion of this rock contains no fossils, nor were the physical conditions such 

 as to have preserved them even if they bad existed. 



" 5. There seem to be evidences of a gradual thinning out of the Primordial 

 sandstone in its far western extension, as also of all the Paleeozoic formations . 

 According to Dr. Owen, the Protozoic sandstones in Minnesota are at least 500 to 

 600 feet in thickness, and in Iowa, Professor Whitney estimates them at from 250 

 to 400 feet. In Tennessee, Prof. Safiford finds several thousand feet of rocks, 

 which he refers to this age, and in Texas, where they seem to be quite well 

 exhibited and to yield a large number of fossils, Dr. Shumard gives them as only 

 about 500 feet. In the Rocky Mountain district they are seldom more than 80 

 feet, and never over 200 feet. Indeed all the primary fossiliferous rocks are but 

 thinly represented there, while the lower secondary formations begin gradually 

 to increase in force until all along the eastern slope we have an enormous devel- 

 opment of the upper Secondary and Tertiary, with an aggregate thickness of 

 from 8,000 to 10,000 feet. 



" 6. So far as we yet know, there is no unconformability in any of the fossil- 

 iferous sedimentary rocks of the northwest from the Potsdam sandstone to the 

 summits of the true Lignite Tertiary. There are proofs of two great periods of 

 disturbance which had a marked influence upon the physical geography of the 

 West. The one occurred prior to the deposition of the Potsdam sandstone when 

 the Azoic or granitic rocks were elevated into a more or less inclined position, 

 and the other and most important period took place at the close of the accumu- 

 lation of the great Lignite Tertiary deposits, when the great lines of fracture 



