IiIAS LIMESTONE AND CLAY. ALUM-SLATE. 



179 



lare generally very regular and flat, and can be easily raised in slabs 

 from the quarry. When the lias beds, with their associated beds of 

 clay, are fully developed, they form a mass of stratified limestone 

 and clay, several hundred feet in thickness, which rests upon the red 

 marl described in the preceding chapter. 



The regularly stratified lias limestone occupies the lower part of 

 the bed, and the lias clay the upper. The lower beds of the linje- 

 stone have often a yellowish white colour, and are called white lias. 

 The blue lias limestone has, generally, a dark smoke-grey colour, a 

 dull earthy texture, and an imperfectly conchoidal fracture : the 

 purest beds contain from 80 to 90 per cent, of carbonate of lime, 

 combined with bitumen, alumine, and iron. If iron enter largely 

 into the composition of this limestone, it forms a lime, which when 

 burned, has the property of setting under water. 



The finer kinds of white lias will receive a polish, and may be 

 used for lithographic drawings. Between the lower lias limestone 

 and the lias clay, there occur, in some situations, beds of sandy lias, 

 with layers of ironstone in nodules: this part of the lias formation 

 has been called marl-stone in some of the midland counties. 



The lias clay frequently occurs in the form of soft slate or shale, 

 which divides into very thin laminae. This shale is often much im- 

 pregnated with bitumen and with iron pyrites, and will continue to 

 burn slowly when laid in heaps with faggots, and once ignited: du- 

 ring this slow combustion, the iron pyrites are decomposed, the sul- 

 phur combines with the oxygen of tiie atmosphere to form sulphuric 

 acid, and this combines with a portion of the alumine in the shale, 

 and forms sulphate of alumine or alum. The alum shale of Whitby 

 in Yorkshire is of this kind ; it has rather a soapy feel, and a slight 

 silky lustre. When the lias clay or alum shale falls, in large masses, 

 from the cliffs upon the sea shore, and becomes moistened by sea 

 water, it ignites spontaneously and continues burning a considerable 

 time. About the middle of the last century, the cliffs of lias clay 

 near Lyme, in Dorsetshire, took fire after heavy rains, and continued 

 burning for several months : at the present time, a hill near Weymouth 

 is ignited by a similar cause ; it is composed of bituminous clay with 

 pyrites, but it is an upper bed in the oolite formation called Kimmer- 

 idge clay. 



Lias clay is impregnated with a considerable portion of muriate of 

 soda, and sulphate of magnesia and soda. The mineral springs of 

 Cheltenham and Gloucester rise in this clay ; but the mineral quali- 

 ties decrease after the springs have been opened some time, which 

 proves that the saline matter is derived from parts of the bed adjacent 

 to the springs, and is therefore soon exhausted. 



The beds of lias clay and limestone are particularly distinguished 

 b}'- the number and variety of the organic remains which they con- 

 tain. Twenty different kinds of ammonites have been discovered in 

 lias, and also nautilites, beleranites, and other species of chambered 



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