﻿Vol. 64.] AND ANTIGORITE-SEEPBNTINES. 169 



pressure be continued, slide one over the other ^ and thus often 

 elude much further crushing.- In the last extreme an antigorite- 

 serpentine may perhaps be produced, but then it will be of the 

 most ' slaty ' type. 



Thus these later observations confirm the opinion founded on 

 work done before the summer of 1905, namely, that while antigorite 

 apparently forms with greater facility from augite than from 

 olivine, pressure is a most important factor in its production,^ and 

 this should come into play before the minerals have undergone 

 much alteration. Enstatite, while forming a mineral rather of 

 the antigorite -type, seems, if un crushed, generally to retain some 

 distinctive characteristic ; so that I think we cannot, as has been 

 suggested, place it nearly on the same level with augite as the 

 parent of the mineral, named with dubious authority from the Val 

 Antigorio.^ 



III. Note on Bowenite. 



The above-described studies led me to re-examine some mis- 

 cellaneous slices of serpentine in my cabinets, among which was 

 one of bowenite given to me by the late General McMahon, soon 

 after writing his paper on that mineral — or rather rock — from 

 Afghanistan/ The microscopic structure, as it differed much from 

 that of the serpentines on which I was then occupied, had escaped 

 my memory, but on looking at it recently, I was surprised at its 

 resemblance" to antigorite. jN'eedless to say that my late friend's 

 description is accurate, though perhaps in the specimen before me 

 flakes are more numerous than fibres,*^ which, however, may be 

 due to my being practised in identifying the former ; also, though 

 I occasionally see three directions of orientation for the flakes, only 

 one seems to me definite enough to be suggestive of a cleavage. In 

 other words, I note here and there a group of flakes — running up 

 to about '02 inch in length — lying parallel with and near one 

 another — suggesting the former presence of a diallage or a bastite. 

 At high (not always right) angles with these are occasional shorter 

 flakes, or groups of such, parallel one with another, while some 

 parts of the slice show 'thorn-structure' on a smaller scale, as 

 described in our paper of 1905. Scattered about the field are a few 

 needles or flakes with a higher refractive index than those forming 

 the groundmass, though not yielding brighter polarization-tints. 



^ The surfaces are so completely slickensidecl that a new fracture must be 

 made to show the rock. 



2 That is the reason why very fair s])eoimen8 of the normal rock can often be 

 obtained from the middle part of an Alpine mass, the outer crush-zones having 

 acted like a ' packing' of shavings or sawdust to the rest. 



3 Quart. Jovxrn. G-eol. Soc. vol. Ixi (1905) p. 714. 



4 Without marked pressure olivine changes into ordinary serpentine more 

 readily than enstatite, and that than augite (perhaps also than hornblende). To 

 produce antigorite pressure seems essential, and this apparently makes augite 

 pass into that mineral more readily than into ordinary serpentine in the other 

 case. 



5 Min. Mag. vol. ix (1890) p. 187. 



*= The flakes, however, not unfrequentlj are rather dagger-shaped. 



