34 RESEARCH IN CHINA. 



A similar plane of unconformity presumably characterizes northern 

 Siberia, where the great plateau of flat Paleozoic strata north of Irkutsk 

 stretches from the Lena to the zone of folds adjacent to the Archean areas 

 along the Jenissei. The base of the Paleozoic is there Lower Cambrian, 

 and the surface beneath it is of the same prolonged cycle as that in south- 

 western Asia. 



An unconformity comparable with that at the base of the Cambrian 

 system occurs throughout Central Asia also, but the super jacent strata are 

 probably Devonian or Silurian. The surface developed on the pre-Cambrian 

 rocks is therefore younger than the Sinian, and there is room to question 

 what features existed in Central Asia during the Sinian period. It is prob- 

 able that the epicontinental sea did not spread over the entire region where 

 the corresponding strata are now absent, although it no doubt covered some 

 part. Whatever land area was exposed at any stage of the advance and 

 retreat of the waters was then being eroded and furnished the sediment of 

 the Sinian strata, which, had there been any considerable height of land, 

 would consist of shaly and sandy deposits. They are, however, limestone, 

 and it is a fair inference that practically all Asia, draining to the Cambro- 

 Ordovician sea, was low and featureless. 



The fact that Asia at the opening of the Paleozoic era was a featureless 

 continent has important bearings. It limits the antiquity of mountain 

 ranges, some of which have been discussed by eminent writers as of pre- 

 Cambrian date, as elevations which have survived since that remote time; 

 and it affords a basis of inference regarding a cycle of inactivity, which was 

 common to other continents as well. 



