4°4 



HISTORICAL GEOLOGY 



it is assumed that the seas which they inhabited were separated by 

 dry land or other barrier to their spread. Here, however, is an 

 opportunity for error, since currents of cold water are favorable for 

 one fauna, while in the warm waters of the same sea, a short distance 

 away, a very different assemblage of animals may flourish. Such 



a distribution has 

 often been reported 

 from the seas of to- 

 day. This objection 

 is not as serious as at 

 first appears, since 

 during much of the 

 geologic past climatic 

 zones were probably 

 not as well established 

 as now. 



(2) The character 

 of the deposits fur- 

 nishes aid in deter- 

 mining ancient shore 

 lines. If a certain 

 formation is a con- 

 glomerate, it is evi- 

 dent that it was laid 

 down at or near the 

 shore, since only 

 strong waves and 

 currents, such as are 

 effective in shallow 

 waters, are able to 

 move coarse gravel. 

 Sandstones are also 

 good indicators of shores, or at least nearness of land. Muds point 

 to shallow seas, while limestones are indicative of seas of wider ex- 

 tent, with more distant shores in which the accumulation of lime 

 carbonate from the remains of shell-bearing and coral-secreting an- 

 imals, and that chemically precipitated, was built up with little inter- 

 mixture of muds and sands. 



(3) 1 he above, as well as other evidences of which space will not 

 permit mention, taken in connection with the distribution of the 



Fig. 374. — Map showing the probable distribution of 

 land and water in the Upper Cambrian. The shaded 

 portions are land. (Modified after Schuchert.) 



