UPPER SECONDARY ROCKS. 
103 
consists of from 80 to 90 per cent, of carbonate of 
lime, combined with bitumen, alumine, and iron. 
If iron enters largely into the composition of this 
limestone, it forms a lime when burned, which has 
the property of setting under water.* The finer 
kinds of lias receive a polish, and are used for lith- 
ographic drawings. The lias clay or marl is often 
much impregnated with bitumen and iron pyrites, 
and will burn slowly when laid in heaps with fag- 
ots and ignited. By this process the sulphur in the 
pyrites is decomposed, and combines with the ox- 
ygen of the atmosphere, and with a portion of the 
alumine in the shale, and forms sulphate of alu-^ 
mine or alum. This variety of shale, when moist- 
ened with salt water, ignites spontaneously ; and 
Bakewell states that it is not an unusual thing for 
cliffs of lias in England to take fire after heavy 
rains, and continue burning for several months. 
Lias clay is impregnated with a considerable por- 
tion of muriate of soda, and sulphate of magnesia 
and soda. This formation is particularly distin- 
guished by the number and variety of the organic 
remains which it contains. Besides an immense 
variety of fossil fish, with the form of the bodies 
and scales well preserved, we find imbedded in it 
the skeletons of enormous animals allied to the or- 
der of hzards or saurians. The bones and teeth of 
reptiles have sometimes been found in the new red 
sandstone ; but in the lias and the strata above it, 
the bones of saurian animals are so numerous and 
of such vast size, that the epoch in which these 
strata were deposited has been called " the age of 
reptiles." The characteristic fossils of the lias have 
not hitherto been discovered in this country, and 
the rock itself is believed to be wanting. 
* Mr. Bakewell states that the property of setting under 
water may be communicated to any kind of hme, by an admix- 
ture with burned and pulverized ironstone. This is well wor- 
thy of trial. 
