CHAPTEK X. 



THE LIASSJC PERIODS. 



THE composition of this large succession of clays, limestones, 

 sands, sandstones, ironstones, and jet-bands is much alike through- 

 out England, though all parts of it are liable to much variation 

 in thickness. As already observed, the lowest part of the lias, as 

 it was understood by Smith, Conybeare, and Lonsdale, has now 

 been regarded as distinct enough to bear the separate name of 

 Rhsetic. We shall however find it useful, and indeed unavoidable, 

 to keep it in memory while treating of the lias, because the physical 

 history of the earlier deposit is inseparably united with that of 

 the later. Another deviation from the old classification of Smith 

 has been made. The sand which hung over his own house near 

 Bath, and into which he drove curious tunnels for dairy and other 

 uses, giving it the name of ( Sand of the Inferior Oolite,' has been 

 pretty generally transferred to the liassic dominion. We may re- 

 present these changes in a diagram : 



INFERIOR OOLITE. 



SMITH. 

 Sand of the inferior oolite. 



Upper lias clay. 

 Marlstone. 

 Lower lias clay. 

 Blue lias limestone. 

 White lias. 

 Black marls, &c. 



GEOLOGICAL SURVEY. 



Sand of the upper lias. 



Upper lias clay. 



Middle lias. 



Lower lias, divided into several 



stages. 



Rhsetic beds. 



RED | MARLS, &c. 



Including the group of Westbury shales, as constituting a lower 



