LATER PROTEROZOIC. 7 



history of Asia and North America, which extends to successive events of 

 erosion, deposition, and deformation. The general relations to the Archean 

 and neo-Proterozoic are similar in both continents, and the effects may well 

 have been due to a general terrestrial cause which became active at about 

 the same times, in regions remote from one another. 



Before discussing the occurrence elsewhere in Asia of pre-Cambrian 

 rocks, which may be equivalent to the Wu-t'ai, I describe the next younger, 

 the Hu-t'o system. 



HU-T'O SYSTEM (NEO-PROTEROZOIC). 



All of the rocks of the Hu-t'o system are sedimentary strata : conglom- 

 erate, quartzite, shale, and limestone, which resemble the unmetamorphosed 

 Paleozoic rocks more nearly than they do the Wu-t'ai schists. The physi- 

 cal events which intervened between the close of the Wu-t'ai period and 

 the beginning of the Hu-t'o involved greater changes and probably longer 

 time than those which occurred 'after the Hu-t'o and before the Sinian; 

 but the presence of a rich fauna in the Sinian seas distinguishes that period 

 from the preceding time, during which the life forms, though probably 

 numerous, did not generally become fossil. The nearest relations of the 

 Hu-t'o system are with the Belt terrane of Montana, and it is probable that 

 pre-Cambrian fossils* such as have been found in the Belt may eventually 

 be discovered in the Hu-t'o. 



The Hu-t'o rocks occur in typical development in the district of Wu- 

 t'ai-ihen, in northern Shan-si, where they form the southwestern part of the 

 Wu-t'ai-shan and the hills about the Northern I/oess Basins as well as along 

 the Hu-t'o river. They occupy a broad synclinorium and lie between the 

 Wu-t'ai schists and Sinian strata, in unconformable relations to both 

 systems. Although the contact with the underlying Wu-t'ai series must 

 be extensively exposed in the mountains southwest of the upper T'ai-shan-ho 

 and also north of the Loess Basins, we did not see it. It was covered by 

 loess in each of the sections along which we crossed it, and we did not recog- 

 nize the distinct position of the unfamiliar Hu-t'o rocks in time to search 

 for the contact. There can, however, be no doubt of an unconformity 

 between the schists of the Wu-t'ai and the little-altered, slightly slaty beds 

 of the Hu-t'o. After the Wu-t'ai sediments had been deposited they were 

 folded, depressed by folding or subsidence to a notable depth, intruded by 

 large igneous masses, and deformed under great pressure, so that their 

 original structure was replaced by schistosity and their constituents were 

 recrystallized. The Hu-t'o rocks suffered none of these changes. They 

 were deposited only after the schists had been exposed by uplift and deep 

 erosion. Obviously an interval of the first magnitude intervened. 



*Pre-Cambrian fossiliferous formations, C. D. Walcott, Bull. G. S. A., vol. x, p. 199, 1899. 



