NOTES ON THE GEOLOGY OF CLEVELAND. 



SO 



frequent occurrence on the coast at Runswick Bay. Here, it is 

 scarcely necessary to remind the reader, is a most marvellous pro- 

 fusion of Ammonites, Belemnites, &c, and an abundance of the now 

 familiar remains of Ichthyosauri and Plesiosauri. An excellent 

 section of these rocks, and a delightful field for geological observa- 

 tion, will be found in the quiet and picturesque bay of Runswick. 

 Above this jet-rock are about eighty feet of shale with ferruginous 

 nodules, irregularly interspersed, and for the most part aggregated 

 round Ammonites and other organic bodies. Being very compact 

 and argillaceous, they closely resemble some of the clay-ironstones of 

 Derbyshire, and claim a similar, though far from a synchronous 

 origin. Above this shale, again, we reach the fine argillaceous 

 deposit so well known by the name of " alum- shale," which is from 

 eighty to one hundred feet in thickness, and has been worked for the 

 manufacture of alum since about the year 1594, when alum-works 

 were first introduced into the district of Guisborough, by Sir Thomas 

 Chaloner, where these shales have ever since been more or less ex- 

 tensively wrought. Scattered up and down through the whole 

 length of Cleveland, we find traces, in vast heaps of calcined shale 

 and large excavations, of abandoned alum- works, as at Carlton Bank, 

 Kirby Bank, above Great Ayton and several other places, whilst we 

 find in present operation the works near Guisborough, at Boulby, 

 and at Kettleness, The process of manufacture is simply as follows : 

 the decomposition of the shale is accelerated by being burnt in large 

 heaps by the manufacturer, who avails himself of the carbonaceous 

 character of the shale, and the sulphur of the iron is changed into 

 sulphuric acid, which forms a sulphate of iron and alumina ; by sub- 

 sequent processes of evaporation, the sulphate of alumina is purified, 

 and potash is added to render the salt cry stalliz able. 



Above the alum-shale is a stratum about twenty feet in thickness, 

 which is usually termed the " Cement-rock," from its containing 

 numerous calcareous nodules, which are used in the manufacture of 

 Roman cement. These nodules will, for the most part, be found to be 

 an aggregation round an organic nucleus ; and by baking them until 

 they divide, I have frequently procured most beautiful specimens of 

 fossil Ammonites. Above this is found, in a perfect section, a deposit 

 of about four feet of indurated clay known as the "soft jet-rock," from 

 its containing a quantity of jet, inferior however to that in the hard 

 jet-rock, of which T spoke before. This rock, in opposition though 

 it be to many, I shall make the highest one and the conclusion of the 

 Lias formation, the upper division of which, as I have described it 

 above, may in summary be given as follows : 



ft. in. 



Soft jet rock about 4 0 



Cement-rock „ 20 0 ft. in. 



Alum-shale (used in manufacture of alum) 80 0 to 100 0 

 Alum-shale (not so used, and with many 



nodules) 60 0 to 80 0 



Hard jet rock 18 0 



Hard sandy shale 20 0 to 40 0 



VOL. IV. M 



