J. H. Collins — Cornish Serpentinous Rocks. 365 



tinous mass, well known to tourists as well as to geologists, has 

 been fully described by Messrs. Allport l and J. A. Phillips. 2 



Mr. Allport regards the serpentinous base as being due to the 

 metamorphism of felspar, and this is no doubt the case to some 

 extent, as at Duporth ; however, much of it appears to be due to the 

 alteration of hornblende, itself an alteration product of augite. 



Mr. Allport describes it as follows : — 



" Under the microscope, a thin slice of the intrusive rock exhibits 

 a variegated mass of pale green serpentine and a nearly colourless 

 substance intimately blended together ; imbedded in this matrix 

 there are numerous pseudomorphs after olivine, and irregular plates 

 of unaltered augite, together with minute grains of magnetite scattered 

 here and there through the mass. The pseudomorphs after olivine 

 are of two kinds, consisting either of serpentine or the white sub- 

 stance just mentioned ; both are highly characteristic ; the crystalline 

 forms are perfectly preserved, and they are traversed by veins 

 representing the original cracks so generally formed in this mineral. 



"In one slice the augite is also greatly altered, and it is interesting 

 to compare the two pseudomorphs side by side. 



"Although the ground-mass of the rock exhibits a confused amor- 

 phous appearance both in ordinary and polarized light, we are 

 fortunately not left in doubt as to the nature of at least one of its 

 original constituents. The augite frequently incloses highly charac- 

 teristic pseudomorphs after felspar; some are completely inclosed, 

 while others are only partially imbedded in it ; the mode of occur- 

 rence is, in fact, precisely the same as that observed in the altered 

 gabbro of Costorphine Hill, near Edinburgh, and in several Scotch 

 clolerites described by me on a previous occasion. 3 In many cases 

 the unaltered augite has preserved in the most perfect manner the 

 sharp edges and angles of the felspar prisms; and whenever the 

 latter project from the augite, it may be readily seen that both the 

 inclosed and outlying portions have been converted into precisely 

 the same serpentinous substance as that forming the ground-mass. 

 It should also be noted that, in the case of partially-inclosed prisms, 

 only those sides or ends which have impressed their shape on the 

 augite exhibit a crystalline form, the outstanding portion being quite 

 undistinguishable from the surrounding mass. 



" As it would evidently be absurd to suppose that originally there 

 was no felspar save that inclosed in the augite, there can be no 

 doubt that the original felspathic matrix has been completely meta- 

 morphosed, and that we have in the Menheniot. rock a highly-in- 

 teresting and instructive example of the conversion of an intrusive 

 olivine-dolerite into a mass of imperfectly-formed serpentine." 



The following is an analysis of this rock by Mr. Phillips : — 



Silica 36-60 



Alumina 17'58 



Ferric Oxide 14-98 



Ferrous Oxide 4*52 



1 Quart. Journ. Geol. Soc. 128, Nov. 1876, p. 422. 2 Phil. Mag. 1871. 



3 Quart. Journ. Geol. Soc. vol. xxx. p. 557. 



