224 J. H. Collins — Cornish Serpentinoiis Roclcs. 



of " segregation " and of " selective metamorpliism " in one and 

 the same slide, the result in each case being serpentine.^ 



No. 2, same locality. Here the serpentine is associated with 

 chalcedony and " a yellowish- white matter, apparently kaolin." 



No. 4, same hill, west end. Here there is " a large proportion of 

 serpentinous matter ; so large, indeed, that by burnishing a smoothly- 

 cut surface of the rock with an ivory paper-knife, a fairly good 

 polish can be communicated to it. When we examine a section of 

 this rock under the microscope, we find some of the original glassy 

 basis has to a large extent been converted into serpentine, especially 

 along lines which seem to indicate fluxion-structure. There is in 

 addition to the serpentine a large proportion of kaolin present." 



Nos. 5 and 6, east side, foot of hill. These also contain a very 

 considerable admixture of serpentine ; and the author concludes — 

 " There seems then no doubt that the upper portion of Cant Hill, 

 which has been mapped as * Greenstone,' is really composed of a 

 basic lava of a once vitreous character, but so altered that its original 

 mineral constitution cannot be inferred with precision." ^ Mr. Eutley 

 goes on to speak of the occurrence of serpentine in other eruptive 

 rocks in the locality, and especially at Carlion near Cant Farm. 



The occurrence of kaolin in Nos. 2 and 4 connects them with the 

 Duporth rock already referred to,^ where also very extensive kaolin- 

 ization of felspathic matter has taken place. 



IV. Examples from Devonshire. 



Greston Bridge. — The serpentinous rock of this locality is figured 

 (No. 27) and described by Mr. Eutley in his excellent account of 

 the eruptive rocks of Brent Tor, which is published at a prohibitive 

 price by Her Majesty's Stationery Office. It is very incompletely 

 serpentinized, and no analysis has been published or made so far as 

 I know. A similar rock exists at Dunterton. 



Anstey's Cove, near Torquay. — This is a coarsely crystalline rock 

 of a dark green colour, and consists of augite, small prisms of triclinio 

 felspar, apatite and magnetite. Some of the augite is but slightly 

 altered ; there is a little pale-green serpentine disseminated through- 

 out the mass.* In some parts it is very largely serpentinous, 

 much more so than the specimens which Mr. Allport describes would 

 appear to be. The rock occurs distinctly as a dyke cutting through 

 Devonian rocks. No analysis of the rock has been made to my 

 knowledge. 



1 In a note Mr. Eutley says : " Prof. Bonney, who favoured me with an opinion 

 upon these sections, regards much of the substance which I have here called 

 serpentine as a palagonitic material." But palagonite is a constituent of recent 

 volcanic tufa, and hardly likely to be found in so old a rock as this. Moreover, it is 

 fusible, while tbis mineral, like serpentine, is very infusible. Further, Mr. Eutley 

 says the serpentinous matter has a hardness of 3 or slightly less, while the hardness 

 of palagonite is from 4 to 5. — J. H. C. 



2 Op. cit. p. 397. 



3 See Geol. Mag. August, 1886, p. 362. 



* Allport, " Metamorphic Eocks surrounding the Land's End Mass of Granite," 

 Quart. Journ. Geol. Soc. 1876, vol. xxxii. p. 423. 



