OF THE SECONDARY CLASS. 389 



dary strata may arise from two causes. They 

 may have been destroyed during the lapse of 

 time, in consequence of the ordinary effects of 

 wear, or of other causes respecting which we can 

 only conjecture. There is also reason to suppose 

 that they have often been originally but partial de- 

 posits ; limited to certain districts on the surface, 

 probably of a concave form, and technically 

 termed basins. This latter opinion is supported, 

 partly by the peculiar manner in which the dif- 

 ferent members of a series come successively in 

 contact with the primary strata, the highest stra- 

 tum covering the widest space, and partly, by the 

 different order of succession of the members [a 

 different places, and by the peculiar limited cha- 

 racters of the several deposits. 



