Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 37 



The rocks which immediately surround the insulated 

 masses and ranges of granite, are very numerous; and although 

 they are generally schistose, they are sometimes massive, and 

 in their structure very similar to the granitic. These circum- 

 stances, however, will come under consideration hereafter ; at 

 present, our object is to obtain a knowledge of the composi- 

 tion and internal appearance of the individuals of the schis- 

 tose group. 



The primary slates, which have been longest known, and 

 most frequently described, are gneiss, mica-slate, and clay- 

 slate ; in many places they follow each other in this order ; 

 and thus they were observed by Werner in Saxony and Bo- 

 hemia ; and it was therefore for a long time considered as the 

 only and true order of succession. 



A more extended experience, however, has not only shown 

 that granite may be in immediate contact with either of these 

 rocks, but also that there are many other primary slates, which, 

 in like manner, are not subject to any fixed laws in the man- 

 ner of their association. For our knowledge on this subject 

 we are more particularly indebted to Macculloch, and the 

 French geologists. 



The following are the principal additions which have been 

 made to the primary slates of Werner : viz., quartz-rock, 

 actynolite, hornblende, shorl, chlorite, and talc schists, and 

 steaschist ; and in the 4th volume of the Cornish Geological 

 Transactions some others have been proposed ; so that it may 

 be safely predicted, that as our knowledge increases, this cata- 

 logue will be extended. 



In describing the various schistose groups, that of Cornwall 

 will in the first place demand our attention. It may be 

 divided into two series, the porphyritic and the calcareous ; 

 the former including those rocks which occur next the granite, 

 and contain porphyries and other granitic rocks in the form 

 of regular beds or elvan-courses, and which abound in veins of 

 tin and copper ores ; the latter, comprising those rocks which 

 are more or less remote from the granite, contain no elvans, 



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