CHAPTER El EARLY PALEOZOIC. 



SINIAN SYSTEM (CAMBRO-ORDOVICIAN). 



The name and its application. Sinian was first applied by Pumpelly 

 to the prevailing structural axes of eastern Asia, which trend northeast and 

 southwest.* It was adopted by von Richthofen to designate a series of 

 conformable strata which exhibit folds having the Sinian direction. They 

 are characterized in part by Cambrian fossils, but were believed by him to 

 extend downward below the base of the Cambrian, and at the top to include 

 part of the Ordovician. The term is here used to designate the Cambrian 

 and Ordovician strata to which he applied it, but those limestones which 

 underlie the lowest fossiliferous Cambrian are excluded, after conference 

 and agreement with von Richthofen himself. 



The problem which confronts us in determining the base of the Sinian 

 is inherent in the more or less local nature of an unconformity. Each un- 

 conformity is somewhere represented by continuous, conformable deposits, 

 and the area of unconformity is bounded by areas of conformity. When 

 we pass from one to the other there is difficulty in dividing the con- 

 tinuous series of strata at a plane corresponding to that indicated by the 

 discontinuity in the neighboring series. This condition exists at the base 

 of the Cambrian in certain localities in the United States, where the lowest 

 fossiliferous Cambrian strata are conformably underlain by great thick- 

 nesses of sediments, that accumulated in the depressions from which the 

 Cambro-Ordovician epicontinental sea expanded. Such sediments are by 

 some regarded as pre-Cambrian, by some as the downward extension of 

 the Cambrian. There is no difference of opinion regarding the base in 

 sections where the unconformity intervenes, as is commonly the case. 



In China there is usually an unconformity at the base of the distinc- 

 tive red formation of the Sinian, and Cambrian fossils occur within 100 feet 

 above the contact. Von Richthofen observed this conspicuous break, and 

 we also obtained evidence of it in every section in which we saw the appro- 

 priate contact. But there are sections such as that of the Nan-k'ou pass, 

 northwest of Peking, in which the unconformity was not noted by von Richt- 

 hofen and may not exist. The strata there below the Cambrian are siliceous 

 limestone, equivalent to that which is apparently unconformable beneath 



* Smithsonian Contributions to Knowledge, vol. xv, Geological Researches in China, Mongolia, and 

 Japan, p. 67. Sinian from Sinim, the name applied to China in the earliest mention made of that 

 country in Isaiah. 



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