THE PLAIN OP TRANSGRESSION. 31 



occasions a decided unconformity of structure in the Wu-t'ai district and 

 is represented by an erosion interval, even where there is no known dis- 

 cordance of dip with the Sinian. It is consequently regarded as an interval 

 of sufficient importance to distinguish the Hu-t'o or Nan-k'ou system of the 

 Proterozoic era from the Sinian system of the Paleozoic era. 



PRE-SINIAN UNCONFORMITY. 



The unconformity at the base of the Sinian system divides the Paleo- 

 zoic from the pre-Cambrian. It is a break of the first magnitude, corre- 

 sponding to a period of deformation and erosion, even where the underlying 

 strata are the Ta-yang (Nan-k'ou) limestone of the late Proterozoic. More 

 commonly the subjacent rocks are Archean and the hiatus embraces the 

 much longer times which are in some localities represented by the two 

 Proterozoic systems of strata and the sum of deformations and erosions 

 they have undergone. 



The latest cycle of erosion with which the Proterozoic closed was very 

 complete. The land surface was reduced to a nearly perfect plain, upon 

 which the transgressing sea of Sinian time found few eminences to level. In 

 this statement we agree with von Richthofen as regards the completeness 

 of planation, but differ from him in assigning to erosion the greater part 

 of the work accomplished. In accordance with the prevailing views of the 

 time in which he wrote his second volume of China, he therein held that the 

 major work of denudation was due to marine abrasion. He says: 



The first great occurrence after the episode of folding was an extensive abrasion, by 

 far the most important which is to be demonstrated in the geological history of China. 

 The complete difference between the tectonic movements which occurred before and after 

 the period marked by this boundary suffices to show that a very long time interval lay 

 between the happenings of the fifth and seventh phase, since a portion of the earth's crust 

 which previously had been capable of intense internal movement was so stiffened that it 

 thereafter only changed its level as a whole ; but we obtain a more definite idea of the great 

 length of this sixth period when we consider that enormous mountain masses which covered 

 continental areas were completely swept away. From the fact that formations whose 

 thickness can be measured only by tens of thousands of feet occur in single troughs, we are 

 obliged to draw the conclusion that these are relatively small roots of former folded moun- 

 tain chains, which attained at least the height of the Alps, but then almost completely 

 vanished. We have repeatedly referred to the breakers of an advancing sea, which trans- 

 gressed over the land in consequence of simultaneous mechanical erosion and positive sub- 

 sidence, as the one agent which is able to produce a level surface where there previously 

 existed an extensive mountain range. However much assistance may be given by atmos- 

 pheric influences and the running water of the mainland, this alone is competent to produce 

 an almost even surface of great extent. That force is the only one which we may here 

 postulate. The surf removed not only the folded masses of the ancient formations, but 

 also attacked their foundation, the old gneiss, and developed a plane of abrasion which ex- 

 tends across the remaining portions of the gneiss as well as over the synclines of the younger 



