ALUM SHALE. 131 



calcined ; a process which is much facilitated by the quantity of sul- 

 phur and bitumen in the schist. The rock, indeed, under certain 

 circumstances, is subject to spontaneous combustion, whert suddenly 

 exposed to moisture and the effects of the atmosphere. Some years 

 ago, a considerable part of the cliff, in a small wyke or bay between 

 Runswick and Staiths, fell down and took fire, and continued to 

 burn for two or three years. The upper part of the rock, containing 

 most sulphur, is the most valuable for producing alum, a cubic yard 

 at the top producing as much as five cubic yards at the depth of 100 

 feet. The quarrying of the rock is not carried on to a greater depth 

 than about 90 or 100 feet, as below that depth it is too unproductive 

 to remunerate the proprietor. The calcined shale is steeped in pits, 

 and the lixivium being drained off, is boiled in pans, with a certain 

 proportion of muriate of potash ; and the liquor being conveyed into 

 coolers, deposits the crystals of alum ; which are afterwards washed, 

 and dissolved by boiling, to be re-crystallized in a purer state.* 



The main bed of alum shale abounds with petrifactions, consist- 

 ing of wood, bones, but especially shells, among which the ammo- 

 nites (the well known Whitby snake-stones) hold a conspicuous place. 

 The petrified substances often occur in a detached form among the 

 schtstus, but they arjg more frequently enveloped with a crust, or 

 included in hard nodules. These nodules, which occur in almost all. 

 parts of the aluminous sti^ata, are generally of a kind of blue liiMestbnei- 

 combined with pyrites ; which last often forms the outer crust of the 

 nodules, and also penetrates them in the form of veins. Sometimes 



* For a more full account oT the process of making alum, the history of the aliim works, 

 &c., see History of Whitby and the Vicinity, Vol. II. p. 806—817. See also Mr. Winter's 

 Essay in Nicholson's Philosophical Journal for April, 1810. In that Essay, however, there 

 are a few mistakes. For instance, it is asserted, that both the sandstone and schistus, in the 

 alum district, are traversed by veins intersecting each other at right angles, so that "their 

 masses always appear in the form of solid parallelograms"; when the truth is, that the sand- 

 stone is extremely irregular as to its partings, and the seams of the schislus for the most pait 

 cro4;s each other ohruiuely, dividing the rock into rhomboidal masses. 



