Ch. VII.] THE GRANITIC AND SCHISTOSE ROCKS. 131 



slate appearances which have been likened to some varieties 

 of gneiss : and it is no uncommon occurrence, in all these 

 cases, for the compact felspar base, by the gradual increase of 

 its silica, to become more and more quartzose ; the beds of 

 these slates next the granite being hard and brittle, exhibiting 

 a sharp and splintery fracture, and very little tendency to 

 separate into laminae. Particular references for each of these 

 appearances need not be given, for the same bed, even within 

 very narrow limits, assumes various aspects : indeed, even in 

 the same concretions, the slate may in one part be quartzose, 

 in another micaceous, resembling gneiss, and in a third (but 

 often in adjoining concretions) the texture may be fine and 

 uniform, and as fissile as the slate at a mile distant from the 

 granite. These facts are well illustrated in the extensive 

 horizontal section at Mousehole, and may, in a lesser or 

 greater degree, be observed at all the junctions on the coast 

 of the Land's End district: so that, although there is no 

 determinate order in the arrangement of the varieties which 

 each kind of slate presents when in immediate contact with 

 the granite, yet it is established that the slate, in such a 

 position, always partakes of different characters, arising from 

 alterations in its composition and texture. 



The granitic rocks, also, as they approach the junction, are 

 likewise subject to similar transitions from the most perfect 

 form to a state which, if viewed apart from the main mass, 

 would sometimes 'not be easily recognised as granite. The 

 most common mode of this metamorphosis is effected by the 

 basis of a porphyritic granite becoming gradually very fine- 

 grained, and alternating with layers of the same, in which the 

 crystals of felspar are wanting. When the basis is very fine- 

 grained, it perfectly resembles the compact felspar of elvan- 

 courses; and when, in this state, it comes in contact with 

 granular slate, they can only be distinguished from each othei 

 by the dark colour of the latter, and cannot be perfectly 

 separated by the hammer: sometimes, however, they are 

 distinctly divided by a seam or joint, each rock being con- 



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