78 RESEARCH IN CHINA. 



as the last of the Paleozoic or the first of the Mesozoic, an epoch that could 

 be fixed upon as contemporaneous throughout several continents, or even 

 within one extensive continent? It does not appear that they do. 



Granting that deepening of sea basins would withdraw the continental 

 seas simultaneously if the continents stood fast, it is evident that the lands 

 would emerge soon in one area and only much later in another if the conti- 

 nents were here rising and there sinking, as was the case. The effects of 

 emergence, i. e., erosion or continental aggradation, were therefore not even 

 approximately contemporaneous, for we must bear in mind that earth 

 movements are slow. Denudation and aggradation were among the earliest 

 processes to become active and the longest to continue. Among the Asiatic 

 effects are: that eroded surface which uncovers strata ranging from the 

 Carboniferous down to the Silurian in the western Himalayas;* or those 

 extensive continental deposits of red cross-bedded sandstones and associated 

 shales, which in North China conformably overlie the Upper Carboniferous 

 coal-measures. 



The differentiation of local climates from the preexisting conditions 

 of a general, somewhat uniform climate may reasonably be compared with 

 similar changes during the Pliocene and Pleistocene, following upon the gen- 

 eral mildness of the Tertiary. The Paleozoic-Mesozoic transition presents 

 even more remarkable extremes, such as the wide occurrence and persistence 

 of the Gondwana flora and the development of centers of glaciation in 

 India and Australia.! Regarded as an effect of refrigeration and aridity, 

 the formation of iron oxide in quantity can not have been an immediate 

 result; geographic and meteorologic changes had progressed notably before 

 the deposition of red rocks could have become general. 



Marine organisms, evolving in ocean waters (that in contrast to air 

 constitute a medium which is extremely conservative in maintaining the 

 conditions of life) are long protected against change and also very sensitive 

 to change, especially of temperature. Thus it is not surprising that in 

 some ocean currents where the mildness of Carboniferous seas was main- 

 tained, Paleozoic forms should have lived on, even after their habitat had 

 been invaded by Mesozoic types, as witness the Productus limestone of 

 India; or that elsewhere, on the diversion of warm waters and occupation 

 of their territory by chillier ones, there should be speedy extinction, even 

 of a whole fauna. Even though the endurance of the adult be consider- 

 able, that of the larva is very slight, and the latter is the critical factor. 

 When we thus consider the physical conditions which permit or limit the 

 existence of species, the value of fossils as evidence of contemporaneity is 



*Haydcn: Memoirs Geological Survey of India, xxxvi, pt. I, p. 52. 



t Textbook of Geology, A. Geikie, vol. 11, pp. 1058, 1079, a nd numerous references there given. 



