Ch.flV.] SCHISTOSe GROUP OF CORNWALL. 39 



rocks that are very fissile, all of which have been indiscrimi- 

 nately called clay-slate, notwithstanding they sensibly differ 

 from each other in their external and physical characters, and 

 are respectively associated with distinct suites of rocks. 



These differences of opinion have arisen from geologists 

 having examined the Cornish slates in too cursory a mariner. 

 A more careful scrutiny would have shown that, the rock 

 adjoining the granite is neither greywacke nor clay-slate, but a 

 rock sui generis. It is not meant to assert that this rock is not 

 analogous to any of the innumerable varieties of rocks in other 

 countries, which have been termed clay-slate ; but that it does 

 not correspond with clay-slate, in the ordinary acceptation of 

 this term. 



The rock adjacent to the granite in the western part of 

 Cornwall, in which most of the productive mines are situated, 

 is of a different nature from that similarly placed in the 

 eastern part of the county ; and it is worthy of remark, that 

 the latter not only abounds less in metals, but is also associated 

 with rocks different from those which occur in the former : 

 the distinction of these is therefore important in an economical, 

 as well as a scientific point of view. The former slate appears 

 to be nearly allied to the thonschiefer of Johanngeorgen- 

 stadt : the latter, to the clay-slate described by Mr. Weaver, 

 as occurring in the southern part of .the granitic range in the 

 east of Ireland. 



Before attempting to show the composition of the Cornish 

 slates, now under consideration, it will be well to describe 

 their respective appearance : and, until geologists have de- 

 termined their true nature, and agreed on distinctive appel- 

 lations, the terms cornubianite and proteolite, proposed in the 

 Cornish Transactions, may, for the present, be admitted, in 

 order to avoid needless repetitions. 



Cornubianite, the rock most abundant in the western part 

 of Cornwall, exhibits various shades of dark blue and purple, 

 sometimes of an uniform colour, but occasionally with dark 

 stripes, spots, or patches, on a light blue base : it varies from 



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