1823.] Geology of Devon and Cornwall. 37 



be recapitulated), as to its being a variety of greywacke, which, 

 if that term has any definite meaning, it unquestionably is not, 

 has been at last admitted on all hands to be genuine clay slate. 

 But this appellation perhaps, after all, does not convey a much 

 clearer notion of the real nature and constitution of the rocks 

 included under it than the repudiated greywacke. An opinion 

 on this subject (nearly identical with that which has for many 

 years been my own) is to be found in the very interesting Cata- 

 logue Mineralogique of the Comte de Bournon: " Les parties 

 composantes qui entrent dans la substance du killas sont, le 

 mica vert tres attenue nommee chlorite, le quartz, et le feldspath ; 

 €t les varietes qu'il presente dependent de la maniere dont ces 

 trois parties se reunissent entr'elles." (Bournon Cat. Min. 

 463.) Mr. Hawkins, in a paper written evidently without 

 the knowledge of C. Bournon's work (probably indeed from 

 materials collected before its publication), appears to hold 

 nearly the same view. *' There is much reason (he writes) to 

 consider it {killas) as an intimate mixture of quartz with mica, 

 talc, chlorite, and perhaps, in some instances, with felspar. 

 We may trace the last in those varieties of the slate which, 

 in this country, are contiguous to the granite.* On the other 

 hand, the talcose ingredient of this mixture is more conspicuous 

 in the varieties which occur at a distance from that rock." If 

 that, which I cannot but suspect to obtain as a general 

 law (see Annals , vol. v. p. 189) ; namely, that stratified rocks 

 are in their mineralogical composition only varieties of the 

 crystalline masses with which they are most largely and closely 

 associated y be admitted, we shall have difficulty in recognising 

 in the numerous elvans by which it is traversed, the crystalline 

 analogue of the killas. The substance occasionally termed clay- 

 stone,t might perhaps afford a link in the series connecting the 

 two extremes. At all events, that term, as well as clayslate, 

 has been very vaguely applied, and is in itself ambiguous. A 

 ready means of detecting the mineralogical constituents of these 

 and the like obscure aggregates (if we admit them to be such), 

 would be among the most valuable services which chemistry 

 could render to geology* 



It may be added that the inferior slate occasionally exhibits 

 very remarkable instances of curvature and contortion. The 

 coast of St. Agnes, a spot highly interesting both for the mine- 

 ralogist and geologist, will afford more than one example. 



* This restriction is not universally borne out by facts : it should rather have been 

 stated, that " we may trace the last mo7-e abundantly.''^ The passages connected with, 

 and following those which I have adduced, well deserve the attention of the geologist. 

 There are some statements in the preceding half of the same paper which Mr. Hawkins 

 himself, on recurring to the advances made in geological science since the period at 

 which his materials appear chiefly to have been collected, would probably be the first to 

 cancel or to modify. See especially p. 6. 



f A rock of this character is found associated with the chloritic, though it seems more 

 common in the ampliibolic series. 



