316 On the formation of the 



homogenous composition to the eye, and which splits up into 

 thin lamina, as commonly used for roofing and for writing on. 

 In using the term, therefore, the above argillaceous schist 

 is naturally presented to the mind, and although it does 

 require a little stretch of the imagination to apply this term to 

 the crystalline schists, such as gneiss, &c. yet there is no 

 absurdity involved ; but when we come, like Dr. MacCulloch, 

 to apply the term in a generic sense to a fanciful class of 

 rocks, which are not associated together in nature, and to in- 

 clude under this term the most incongruous rocks, which 

 have not the slightest resemblance to one another, the term 

 then becomes meaningless and absurd ; and accordingly 

 most absurdly has it been applied unto every rock, for which 

 careless and hasty writers could not readily hit upon a 

 name. 



Dr. MacCulloch in Iona, (Western Isles,) describes a rock 

 composed of black mica, in compact aggregation with quartz 

 and felspar, which splits into large slabs, i. e. has a schistose 

 structure. This rock he remarks, polishes nearly black, and 

 therefore most likely the colour of its fracture will be dark co- 

 loured, and the rock therefore will much resemble those slabs 

 of slate worked up in London into square cisterns for water. 



There can be no doubt, that the above is a proper ap- 

 plication of the term mica schist ; but, we err greatly when 

 we come, like some authors,* to apply the term to any aggre- 

 gation of quartz or felspar with mica, in which the mica is in 

 very minute quantity. The rock has no longer a schistose 

 structure even though the parallelism of the mica can be ob- 

 served ; none but the most determined systematist will assert, 

 in such case that the term can be properly applied. On 

 the above grounds I propose, therefore, that we should con- 

 fine the term hornblende slate as follows, to what is indeed 

 only a schistose variety of black granite : — 



Colour. — Black, sometimes a little speckled in appearance. 



Lustre. — Glistering like minute mica, from which it is dis- 



