50 RESEARCH IN CHINA. 



is not, however, probable. Had such later strata accumulated to notable 

 thickness over the several hundred thousand square miles under con- 

 sideration, they must have been removed before the upper Carboniferous 

 was laid down, and they should constitute a notable terrigenous formation 

 between the Sinian and Wu-shan limestones in the region of continuous 

 sedimentation. The Sin-t'an shale occupies this position, and it is but 1,800 

 feet, 550 meters, thick. Considering the extent of the lands from which 

 the sand and clay were derived, the volume of the formation represents 

 a comparatively thin sheet of material eroded. Further, so far as exten- 

 sive, though not comprehensive, observations enable us to judge, the level 

 of unconformity does not vary greatly from a general horizon in the upper 

 Sinian; that is, the limestone remained practically flat and protected, 

 was ultimately bared, and soon after was covered by upper Carboniferous. 

 The changes of condition thus indicated are: (i) shallowing of the sea, such 

 that the bottom, on which limestone had been accumulating, was scoured 

 by marine currents; (2) after an indefinite time, gradual exposure of the 

 limestone in a flat coastal plain, where at any one time a narrow belt was 

 subject to erosion; and (3) progressive deposition of littoral or continental 

 sediments upon the eroded surface. The last occurred not earlier than 

 middle Carboniferous, and the other stages cover Devonian, Silurian, and 

 upper Ordovician. It is possible to introduce a hypothetical stage of depo- 

 sition in this series of events, between i and 2, if we assume that the deposit 

 over the limestone was eroded in the gradual process of exposure; it is, 

 indeed, probable that a thin formation of terrigenous character accumulated 

 as a temporary storage of the material of the Sin-t'an formation in its 

 passage from the crystalline rocks that yielded the quartz and ferruginous 

 clay; but there is no direct evidence of any notable marine formation. 



The date of inception of the epeirogenic movement may be fixed more 

 nearly in the region of continuous sedimentation. Transition beds of 

 shale and limestone follow the Sinian limestone, and among them occurs 

 a stratum which contains a well-characterized middle Ordovician (Trenton) 

 fauna. We may regard these transition beds as marking the close of the 

 general marine condition and the beginning of the period of little or no 

 deposition. In contrast to the areas of unconformity by erosion, the region 

 of conformable deposits presumably remained somewhat deeply submerged, 

 was not subject to marine scour by currents, and was so distant from the 

 lowlands of the time that it received but little sediment. The evidence 

 of this appears through consideration of the Middle Paleozoic strata. 



If the preceding discussion proves trustworthy a very interesting 

 parallel may be drawn with North America. The Cambro-Ordovician 

 transgression there closes with an episode marked by withdrawal of the 



