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 lizards 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 lime, by an admix 

 ture with burned and pulverized ironstone. This is well wor 

 thy of trial. 



