300 Messrs. Bucrland's and Conybeare's Observations on the 



2. Lias. 



The lias consists of strata of blue slaty clay, which^ in the lower part of 

 the formation, alternate with beds of limestone. 



The aggregate thickness of the formation, when greatest within the limits 

 of our district^ may be fairly estimated at 50 fathoms. This is reduced in 

 many places to 25 fathoms, and, near the eastern extremity of the Mendip 

 chain, the lias gradually thins olf until it becomes evanescent. The strata are 

 remarkably parallel, and, on the small scale, are usually horizontal; but, if we 

 continue to trace them over large tracts of country, we find them to be in- 

 clined, and their level to be unequal. Thus at Keynsham, the lowest beds of 

 lias are on a level with the subsided mass of that rock at Bitton^ which is 

 nearly tiiat of the sea ; whilst at Timbury, 7 miles further to the south, to 

 which place the beds of Keynsham are continued without interruption^ they 

 are elevated at least 400 feet higher. 



Where the series is fullest, the two upper thirds of the formation exhibit 

 thick beds of dark-blue slaty clay or marl, alternating with a very few thin 

 strata of calcareous marl-stone. Beneath the clay lie the beds of lias, properly 

 so called, which consist of compact limestone, having an earthy aspect, and 

 smooth conchoidal fracture, and these are separated by thin partings of blue 

 slaty clay. The thickness of any one stratum of limestone rarely exceeds a foot, 

 and more commonly falls short of 6 inches, and the frequency of the alternations 

 of these strata with the partings of slaty clay gives a striped and ribband-like 

 appearance to the rock in all its quarries, and fits its stony beds for extensive 

 use as flag-stone. The upper beds of limestone are usually variable in colour, 

 passing from blue to smoke-grey ; the lower beds are of a pale yellowish 

 white. The latter, from their even and compact texture, have been recom- 

 mended for lithographic printing, and specimens applicable to this purpose 

 have been obtained from the neighbourhood of Corston and Bitton. The 

 stony beds of the lias are about 60 feet in thickness, and occupy almost the 

 whole of the remaining third part of the entire formation. 



Beneath the limestone beds, a third and lowest division of the formation is 

 sometimes found, which is characterized by the presence of black shale. Be- 

 tween the white lias and the black shale, are strata of dark-grey marl, in some 

 of which occur layers of calcareous concretions, irregularly interspersed. The 

 thickness of this bed of marl usually exceeds 12 feet. It is very variable in its 

 character, being sometimes perfectly stony, and then acquiring a granular 

 and coarsely -oolitic structure. In this state it is known by the name of corn- 

 grit, and approaches in its aspect and colour to the beds of the same name. 



