416 



SUCCESSIVE ZONES OF LIAS. 



[Ch. XXL 



at an angle of about 45°, while the incumbent oolitic marls are hori- 

 zontal. 



Tig. 436. 



Fia 



Aticula incequwalvis, Sow. 

 Lower Oolite, and Lias. . 



Avicula cygnipes, Phil. 

 Lias, Gloucestershire and Yorkshire. 



The peculiar aspect which is most characteristic of the Lias in 

 England, France, and Germany, is an alternation of thin beds of 

 blue or gray limestone, having a surface which becomes light-brown 

 when weathered, these beds being separated by dark-colored, narrow 

 argillaceous partings, so that the quarries of this rock, at a distance, 

 assume a striped and riband-like aspect.* 



The Lias has been divided in England into three formations, the 

 Upper, Middle, and Lower. The Upper Lias consists first of sands, 

 which were formerly regarded as the base of the Oolite, but which, 

 according to Dr. Wright, are by their fossils more properly refer- 

 able to the Lias ; secondly, of clay shale and thin beds of limestone. 

 The Middle Lias, or marlstone series, has been divided into three 

 zones ; and the Lower Lias, according to the labors of Quenstedt, 

 Oppel, Strickland, Wright, and others, into six zones, each marked 

 by its own group of fossils. This Lower Lias averages from 600 to 

 900 feet in thickness. 



From Devon and Dorsetshire to Yorkshire all these divisions, ob- 

 serves Professor Ramsay, are constant ; and, from bottom to top we 

 cannot assert that anywhere there is actual unconformity between any 

 two subdivisions, whether of the larger or smaller kind. In the whole 

 of the English Lias, there are about 243 genera, and 467 known spe- 

 cies.f The whole series has been divided by zones^ characterized by 

 particular ammonites ; for while other families of shells pass from 

 one division to another in numbers varying from about 20 to 50 per 

 cent., these cephalopods are almost always limited to single zones, as 

 Quenstedt and Oppel have shown for Germany, and Dr. Wright for 

 England. J 



As no actual unconformity is known from the bottom of the Lower 

 to the top of the Upper Lias, and as there is a marked uniformity in 

 the mineral character of almost all the strata, it is somewhat difficult 



* Conyb. and Phil., p. 261. 



f Ramsay, Geol. Quart. Journ., vol. xx. p. 50. 1864. 



i Dr. Wright, ibid., vol. xvi. p. 10. 1859. 



