40 EXPLANATIONS. 



that cephalopods may occur in the first fossil 

 bands in the places which have been examined in 

 England, and yet remains of inferior animals may 

 be found by themselves on the same or a lower 

 level in some as yet unexplored place not far off; 

 so that a time-interval may there appear to allow 

 for a progressive development. Such seems but 

 a reasonably cautious surmise, when we are told 

 by a high authority, that there are " detached 

 Silurian districts in England, presenting particular 

 changes and modifications, arising from difference 

 of depth, and the variety of currents, and chemical 

 combinations in the seas in which they were 

 formed;" and that, "in consequence of this 

 variety of physical condition, there is a correspond- 

 ing diversity in the traces of organic life in each situa- 

 tion.'''' * What, however, places the matter beyond 

 doubt is, that in North America, where the 

 early stratified rocks are even more amply de- 

 veloped than with us, the highest invertebrated 

 forms do not appear at the first. In the earliest 

 ascertained fossiliferous strata, the Potsdam Sand- 

 stone, the only fossils are lingula (a brachiopodous 

 moUusk) and fucoids. In the next, the Calci- 

 ferous Sandrock, are fucoidal layers, encrinital 



* Professor Phillips, British Association, 1845. Athenaeum's 

 Report. 



