226 C. £). WALCOTT I'RE-CAMBKIAN FOSSILIFEROUS FORMATIONS 



Upper Huronian is in all respects like the upper series of the original Huronian, 

 and can be correlated with it with a considerable degree of certainty. Above the 

 Upper Huronian is the great Keweenawan series, estimated at its maximum to be 

 50,000 feet thick, although its average thickness is much less. Its lower division 

 consists largely of basic and acid volcanic flows, but contains thick beds of inter- 

 stratified sandstones and conglomerates, especially in its upper part. The upper 

 division, 15,000 feet thick, is wholly of detrital material, which is largely derived 

 from the volcanics of the same series. 



" The unconformity which separates the Lower Huronian from the Upper Huro- 

 nian and that which separates the latter from the Keweenawan each represents an 

 interval of time sufficiently long to raise the land above the sea, to fold the rocks, 

 to carry away thousands of feet of sediments, and to depress the land again below 

 the sea— that is, each represents an amount of time which perhaps is as long as 

 any of the periods of deposition themselves. In parts of the region the lowest 

 clastic series rest unconformably on the fundamental complex, but in certain areas 

 the relations have not been ascertained. The upper of the three clastic series, the 

 Keweenawan, rests unconformably below the Cambrian." 



PRE-CAMBRIAN SEDIMENTARY ROCKS IN UTAH, NEVADA, CALIFORNIA, AND 



BRITISH COLUMBIA 



Beneath the lowest horizon carrying tlie Olenellus fauna of the Cor- 

 dilleran region of the United States and British Colunihia there is a 

 great series of silicious slates, sandstones, and occasionally thin bedded 

 limestones in which no trace of life has been discovered. What their 

 relations are to the Belt series of Montana and the Grand Canyon series 

 of Arizona is unknown, as in no locality have they been seen in contact. 

 A somewhat full description of these series is given in Bulletins 81 and 

 86 of the Geological Survey and in the Tenth Annual Report, pages 

 549-552. 



\ Whether the fauna that existed at the time of the deposition of the 

 pre-Olenellus beds under consideration has the character of that of the 

 Cambrian is conjectural. It is not improbable that traces of it will be 

 found when the great sections exposed along more than a thousand miles 

 of outcrop are carefull}^ examined by experienced collectors. 



Occurrence of Fossils 

 strata of determined pre-cambrian age 



An investigation of the reported discover}^ of fossils in pre-Cambrian 

 formations leaves onh^ three undoubted instances where it can be said 

 that the stratigraphy shows the strata to be of pre-Cambrian age and 

 the traces of fossils are such as to prove their organic origin beyond a 

 reasonable doubt, namely, the Grand Canyon terrane of Arizona, the 



