234 CORRELATION OF THE KARROO SYSTEM 



deposited in the sea. Marine forms of past ages are 

 as a rule better known than the land and fresh-water 

 organisms that lived contemporaneously with them, be- 

 cause the former have been preserved in far greater 

 numbers than the latter ; this can easily be explained 

 when we consider the very small chance that land 

 animals and plants have of being covered up by mud or 

 other sediment before decomposition destroys them. It 

 is only near the mouths of large rivers laden with silt 

 and in regions of internal drainage, i.e., without outflow 

 to the ocean, that such remains are likely to be en- 

 tombed ; and, unless the areas in question are gradually 

 sinking, and thus allow the accumulation of much sedi- 

 ment, the deposits formed will probably soon be swept 

 away. The Karroo formation is a very remarkable ex- 

 ample of a thick group of beds laid down in fresh water, 

 or at any rate not under the sea, and in it are preserved 

 scanty remains of a very long succession of animals and 

 plants which lived mainly on land. It is a generally 

 recognised fact that animals and plants that live on the 

 land and in fresh water vary more in different regions 

 than the marine forms in the sea round the shores of 

 the same countries, because the conditions of life are 

 subject to greater changes and there is on the whole 

 less freedom of movement in the case of the inhabitants 

 of the land. This results in the land and fresh-water 

 species having a narrower distribution than the marine, 

 and hence in their being of less general use than the 

 latter for the purpose of correlating distant strata. 

 Another unfortunate circumstance is that the only avail- 

 able plant remains cannot so certainly be assigned to 



