NATURAL HISTORY OF CREATION. 57 



specimens of this system show a remarkable freedom 

 from those disturbances which result in the interjection 

 of trap ; and they are thus defective in mineral ores. In 

 some parts of England the old red sandstone system has 

 been stated as 10,000 feet in thickness. 



In this era, the forms of life which existed in the 

 Silurian are continued : we have the same orders of 

 marine creatures, zoophyta, polypiaria, conchifera, Crus- 

 tacea ; but to these are added numerous fishes, some of 

 which are of most extraordinary and surprising forms. 

 Several of the strata are crowded with remains of fish, 

 showing that the" seas in which those beds were deposited 

 had swarmed with that class of inhabitants. The inves- 

 tigation of this system is recent; but already* M. Agassiz 

 has ascertained about twenty genera, and thrice the 

 number of species. And it is remarkable that the 

 Silurian fishes are here only represented in genera ; the 

 whole of the sjyecies of that era had already passed away. 

 Even throughout the sub-groups of the system itself, the 

 species ai-e changed ; and these are phenomena observed 

 throughout all the subsequent systems or geological eras; 

 apparently arguing that, during the deposition of all 

 the rocks, a gradual change of physical conditions was 

 constantly going on. A varying temperature, or even a 

 varying depth of sea, would at present be attended with 

 similar changes in marine life ; and by analogy we are 

 entitled to assume that such variations in the ancient 

 seas might be amongst the causes of that constant change 

 of genera and species in the inhabitants of those seas to 

 which the organic contents of the rocks bear witness. 



The predominating fishes of this system, and the only 

 ones which (as far as fossils show) existed for some ages, 

 are arranged by M. Agassiz in two orders, wdth a regard 

 * June 1842. 



