OF THE ARCHEAN. 25 



along parallel planes. It is commonly recognized that they acquired these 

 characters when deeply buried beneath masses, which subsequently were 

 eroded in consequence of exposure above sea-level. 



To pursue this thought would lead too far afield in the direction of 

 theory. Adhering to the facts, we may consider the leitlinien, or axial 

 trends, of the Archean in Asia as developed by Suess.* They maintain 

 two principal courses, the one known as the Saian direction striking east- 

 southeast or southeast; and the other, the Baikal direction, east-northeast. 

 Trends having these bearings meet south of Irkutsk, where they were first 

 distinguished by Tchersky,f and there inclose on the south the amphi- 

 theater of Irkutsk. By assembling the geologic observations for all northern 

 Asia, Suess has traced these axial trends and shown that they characterize 

 an immense area of eastern and western Siberia. The peripheral ranges 

 extend from the island of Sakhalin across the Amur into northern Mongo- 

 lia, in the Baikal trend, and thence in the Saian direction through the Saian 

 mountains to the Ob. This structure is not only of vast extent; it is also 

 fundamental, as well through its antiquity as through its nucleal position. 

 About it are arranged the other structures of Asia. They are pendant from 

 it, as Suess happily says, like garlands. 



Succeeding the Archean, in the sense given that term in these volumes, 

 the next oldest rocks of which we have definite knowledge are the Wu-t'ai 

 schists of northern Shan-si. They are early Proterozoic. They probably 

 are not younger than many terranes classed as Archean in the broader 

 sense of that term commonly employed by writers on Asiatic geology, for 

 similar strata have not, as a rule, been sharply distinguished from the basal 

 gneisses. 



The Wu-t'ai schists are distinct sediments, which represent familiar \ / 

 conditions of erosion in a sequence that is generally recognized among 

 later deposits. Unfortunately our interpretation is in a measure balked 

 by uncertainty of the exact stratigraphic succession and ignorance of the 

 former extent and distribution; but I proceed on the basis of our present 

 understanding. 



According to that, the lower Wu-t'ai strata are heterogeneous, quartz- 

 ose and clayey in the lower part, more argillaceous in the middle, calcare- 

 ous above; and this lower series is unconformably succeeded by a great 

 thickness of shale which carries a basal conglomerate of large quartz and 

 quartzite pebbles. Regarded as products of erosion the strata have a 

 perfectly normal sequence, which corresponds to a complete erosion cycle 



*La Face de la Terre, vol. in, chapter HI, Le Fatte Primitif. Summary on page 138. 

 fl. D. Tchersky. Sur la tectonique des montagnes de la Siberie Orientale. Trav. Soc. des Natura- 

 listes, St. Petersburg, xvn, No. 2, 1886. Proces-verb. pp. 52-58. 



