4 RESEARCH IN CHINA. 



WU-T'AI SYSTEM (EO-PROTEROZOIC). 



Rocks of the Wu-t'ai system constitute much of the Wu-t'ai-shan, a 

 short but high mountain range in northern Shan-si, longitude 113 30' E., 

 latitude 39 N. Von Richthofen crossed the mountains in December, 

 1871, and clearly observed the existence of a series of strata which he had 

 not previously seen in thewcourse of his extended journeyings; nor are they 

 elsewhere known in equal mass. During the stormy December weather 

 which he met the conditions were most unfavorable for observation, and 

 his route crossed only a part of the Wu-t'ai rocks, that mass of chlorite 

 schists which we consider the upper part and call the Si-t'ai formation. 

 The lower series of mica schists, quartzites, and marbles he did not see. 



Von Richthofen placed the Wu-t'ai system in the Huronian, using 

 the latter term, as was commonly done thirty years ago, to suggest pre- 

 Paleozoic metamorphic rocks of green color.* He does so with reserve, 

 however, and the stricter usage of the term as it is now adoptedf does not 

 permit us to maintain an exact correlation. Yet there is a certain paral- 

 lelism in stratigraphic position and history, as well as likeness in rocks, 

 which invites comparison of the Wu-t'ai and the Huronian, as will appear 

 in stating the facts as we now understand them. 



Our own opportunities for study of the Wu-t'ai system were limited 

 practically to a single section and a few isolated localities. In volume i 

 we have stated our inference regarding the stratigraphic sequence and 

 structure as provisional only, and we commend to some future investigator 

 the almost unequaled exposures of ancient sedimentary and igneous rocks 

 which represent one of the earliest intelligible records of Asiatic history. 



A provisional classification of the Wu-t'ai and limiting systems in the 

 type locality is as follows: 



Hu-t'o system Tung-yu limestone 1 s , ates _ limestonei and quar , z it e . 



(Neo-Prolerozoic) T6u-ts'un slates 



Unconformity 



I Si-t'ai series Chiefly chlorite schist; quartzite conglom- 



erate at the base. 

 Unconformity 

 .. u-l >. ,,,.,.^,,1 



(Eo-Proterozoic) 1 Nan-t'ai series Siliceous marble, jasper, quartzite, and 



schist. 



Unconformity 



L Shi'-tsui series Mica schists, gneiss, magnetite quartzite, 



and basal feldspathic quartzite. 



Unconformity 

 T'ai-shan complex Basal complex of varied gneisses and 



(Archean) younger intrusives. 



A brief account of the successive elements of the Wu-t'ai section fol- 

 lows (see atlas sheet DI and plate xvm, volume i, part i). 



*China, vol. n, ! p. 377. 



t Report of International Committee, Chicago Journal of Geology, vol. xin, 1905. 



