106 LITHOLOGICAL GEOLOGY. 



waters, from those of cold, whether at the surface or in the deep ocean 

 where oceanic currents make differences of temperature ; those of 

 warm or dry lands, from those of cold or wet ; those of clear, open 

 seas, from those of muddy waters or near muddy seashores ; those of 

 rocky bottoms, from those of muddy ; etc. Hence, an ancient rock 

 made in a clear sea, as a limestone, will necessarily contain very dif- 

 ferent fossils from a rock that was made of mud, although they were 

 formed at the very same time, in the same waters, and within a hun- 

 dred miles of one another. Even a hundred yards may be all that 

 separates widely different groups of species. Again, a rock made in 

 fresh waters will differ in its fossils still more widely from that made 

 synchronously in salt waters ; a rock made in shallow waters, from one 

 made at great depths ; a rock made in the tropics, from one made in 

 the temperate zone or the arctic, provided the zones at the time of the 

 making differed as they do now in climate. Hence, a very considerable 

 difference in the fossils of rocks is consistent with their being contempora- 

 neous in origin. 



2. The difference in the time at lohich species or groups of species of 

 different regions have become extinct. In one region, changes may 

 cause species or genera (or higher groups) to disappear, while, in 

 another, subjected to the same conditions or causes of catastrophe, the 

 same species, or at least the same genera (or higher groups), may con- 

 tinue on through another period. Genera or Families may become 

 extinct sooner on one continent, or part of a continent, than on 

 another ; or in one ocean, or part of an ocean, than in another. 



Catastrophes may affect the borders of an ocean or shallow seas, 

 that do not reach the greater depths. Fossils of the group called 

 Cystids occur only in the older rocks of the globe, and were supposed 

 to have become extinct at the time of their disappearance as fossils ; 

 but recently they have been found in the depths of the Atlantic ocean, 

 a region not reached by the agencies of extermination that swept from 

 time to time over the continental seas. It was formerly supposed that 

 no species that is now alive existed anterior to the Tertiary ; but, in 

 the same deep ocean, one living mollusk has been found that is 

 supposed to date back to the Cretaceous or chalk era. 



3. The difference in the time at which species or groups have begun 

 to exist in different regions. The several continents may not have 

 been exactly parallel, in all the steps of progress in the life of the 

 globe, certain families commencing a little earlier in one than in 

 another. Again, one continental sea or region may have received 

 some of its species by migration from another, long after their first ap- 

 pearance. Here. is a source of doubt, due on one side to special con- 



