Mineralogy and Geology of Nova Scotia. 587 



are found in whinstone, now scarcely distinguishable from the 

 whin, but which have probably had a portion of hornblende 

 and silex introduced into them since they were embedded in 

 their present position, which is the cause of their giving a dark 

 sandy lime. 



Another kind, which will not cleave, often forms layers of 

 considerable size : it is as hard and fine-grained as the lime- 

 stone, contains a larger portion of pyrites, but will not burn to 

 lime, and is usually encrusted with rust. Upon working a 

 few feet into the rock, this variety sometimes changes into a 

 stone resembling the Norway rag, with an undulating grain 

 like that of wood, and coated on one side with a thin layer of 

 fine alum slate. I have seen a single instance in which a 

 cylinder of this slate, composed of concentric layers of grit 

 about as thick as the grains of ash, and surrounded with a 

 thin bark of alum slate, formed such an exact representation 

 of a log of wood, that it might have been mistaken for a petri- 

 faction, had no more of the same variety in a different form 

 been seen. The specimen marked alum slate is taken from 

 the cylindrical block above described. Although fine-grained 

 alum slate like this specimen is very common in Halifax, I 

 have not seen a vein more than 1 in. in thickness. When 

 first broken from the rock, it will mark paper like black lead ; 

 but it soon becomes hard by exposing it to the air. But there 

 are other varieties, in very broad veins, which will yield alum. 

 One rule may be given to distinguish them all. Every slate 

 which gives a blue mark when rubbed upon a piece of the 

 same kind, will, by skilful management, yield considerable 

 alum. The property of marking blue seems to originate in 

 a partial decomposition of the slate itself, that occurs during 

 the decomposition of the pyrites. In most situations where 

 peat bog rests upon slate, shallow beds of blue clay occur, 

 apparently formed from slate decomposed by the peat. This 

 is a very tenacious clay, dries hard in the sun, but crumbles 

 to dust if burnt or long exposed to the air. From this clay 

 alum can be made with less expense than it can from slate. 

 Should any one be disposed to try the experiment, it would be 

 necessary to observe that the substances which yield alum will 

 also generally yield copperas ; that copperas is composed of 

 iron, dissolved in sulphuric acid; that alum is composed of 

 pure clay or alumine dissolved in sulphuric acid mixed with a 

 portion of fixed or volatile alkali (in most specimens of alum, 

 with both); that, if alkaline salt should be added to a solution 

 of iron in sulphuric acid, a portion of the iron would be in- 

 stantly separated, but that the acid dissolves clay most readily 

 when mixed with a certain proportion of alkali. Were alum 



