Tol. 55.] SERPENTINE A'SB ASSOCIATED EOCKS I.V ANGLESEY. 285 



may form irregular and sporadic patches. In other specimens, it is 

 apparently deposited mainly within one mineral constituent. While 

 this may be olivine, in several examples the rock, judging from 

 structures still perceptible, includes two varieties of enstatite, one 

 richer in iron than the other. 



The original rock, as pointed out by one of lis, has been in 

 some cases a dunite^; in others, where enstatite has become an 

 essential constituent, it was a saxonite. Sometimes, indeed, the 

 rock may have consisted wholly, or almost wholl}-, of that mineral, 

 as in the Cruglas boulder.^ A monoclinic pyroxene also has 

 been sometimes present, in which case the rock was originally a 

 Iherzolite, as, for example, south of Penrhyn-Fadog Inlet. 



The minerals are associated in different ways : — (a) Sometimes 

 the rock has a granular holocrystalline structure ; {h) Sometimes 

 it shows lustre- mottling, generally in enstatite or diallage (pp. 290, 

 291); (c) Sometimes the augitic constituent is scattered about in 

 the serpentine in larger or smaller grains. The larger are generally 

 diallage, forming irregularly-distributed and sometimes associated 

 patches. The smaller occur in grains of various shapes, but espe- 

 cially in rather elongated irregular prisms, the latter commonly 

 lying parallel and extinguishing simultaneously, thus siinulating a 

 pegmatitic arrangement. In some examples, the structure might 

 be regarded as a result of corrosion, but in others it more resembles 

 a case of intercrystallization. The serpentinous constituent for the 

 most part appears to have replaced an enstatite, thus we think 

 that an intercrystallization of the two minerals is the more probable 

 explanation^ ; (d) In other specimens the pyroxenic constituent has 

 })assed into a fibrous mineral : this generally forms a matted mass, 

 often occurring in clear colourless patches, which contrast with the 

 normal green or yellowish-green serpentine around them. Some- 

 times, however, the patches are dull brown, as if darkened by a 

 dusty deposit, which appears in some specimens to be in part at 

 least a carbonate. These, however, generally indicate some tendency 

 towards a vanolitic character. 



(2) Yariolitic Serpentine. 



In sections for the microscope, if at all thick, the spherules and 

 the matrix are usually contrasted in colour, the one being generally 

 -whitish, the other rich green or yellowish-green : the latter, even in 

 a thin slice, retaining a greenish tinge, though it may be pale. In 

 most slices granules or crystalline grains of opaeite are scattered, 

 sometimes very thickly, over the ground, but are almost or even 

 wholly absent from the spherules or aggregates. We find, however, 

 in specimens irom one mass well-developed crystalline grains of 



^ Quart. Journ. Geol. Soc. vol. xxxvii (1881) p. 45. 



'■" Other examples are discussed in § IV, p. 291. 



^ In certain slides a parallel intergrowrh of the two forms of pyroxene 

 seems to be exhibited ; see Rosenbusch's ' Microsc. PhTsiogr. of Rock-making 

 Minerals ' [trans!. J. P. Iddhigs] 4th ed. (1898) p. 209 & pi. xvi, fig. 5. 



