Vol. 55.] SERPEN-TINE AN'D ASSOCrATP:D ROCKS IN ANOLBSEY. 287 



structure may bo found. Under the microscope, both appear 

 to be composed of serpentine of the ordinary character with a 

 dolomitic mineral, and a few grains or grp.nules of iron-oxide. In 

 the first type some of the serpentine tends to be streaky, and thiS; 

 is predominant in one part of the slice. Here the carbonate more 

 often occurs in granular patches in which flakes of serpentine, 

 variable in amount, are intercrystallized, and these patches once or 

 twice are faintly radial in structure. In the second type this 

 structure is often very well defined, the spherulites showing a black 

 cross. These sometimes have a border, more rarely also an inner 

 ring, of minute flakes of serpentine ; and occasionally a little serpen- 

 tine or actinolite seems to be intercryotallized with the carbonate 

 (PI. XXIII, fig. 3). Here, then, while the serpentine in parts 

 of the slice continues fairly normal, the other constituent, that 

 which has become spherulitic,^ seems to be almost entirely replaced 

 by a carbonate. The spherulites in one rock north of Cerig-moelion 

 are similarly replaced by a carbonate, but here the spherulitic 

 structure is generally very indistinct, the only good examples in 

 the microscopic slices consisting wholly of carbonate,'^ in which case 

 of course it may be of comparatively late origin. 



Thus in these varioles, their frequently subangular shape, their 

 contrast with the adjacent serpentine, the more or less definite 

 directions in which they are elongated, the uniform character 

 of their fibres, all suggest that they have taken the place of some 

 mineral distinct from that which has given rise to the serpentine. 

 Some variety of augite or hornblende seems the most probable, 

 especially one poor in iron and free from alumina, of which a 

 carbouRte might bo a decomposition-product. The original magma 

 in solidification may have separated into parts corresponding with 

 olivine and pyroxene, the one afterwards ])roducing the (more; 

 abundant) serpentine, the other the fibrous masses. As the latter 

 are sometimes without definite shape, or form bands or streaks 

 indicative of a fluxionul structure, they probably did not pass beyond 

 the stage of incipient or ill-formed crystals or grains, now represented 

 by the varioles. 



In certain of the rocks, a lustre- mottling of enstatite or horn- 

 blende is produced by rounded crystals, which on microscopic 

 examination exhibit faintly radial borders. Boulders near Graig- 

 fawr show a similar structure. Sections have been prepared from 

 three separate blocks. Their difiPerences, though interesting as 

 bearing on the process of mineral change, are only varietal. The 

 original minerals evidently were a pyroxene (for the most part 



^ One specimen frora this quarry has a vein of a pale g[reyish-green mineral, 

 compact and uniform, almost waxy-looking, with a hardness not less than H, 

 On examination with high powers, the slice is found to be clear and colourle?R, 

 speckled with very minute granules irregularly grouped around clearer patches. 

 The latter appear to ba composed of an aggregated flaky mineral with very low, 

 polarization-tints, the granules giving brighter colours. The hardness is too 

 high for serpentine or even pseudophito, wliile the microscopic structure is n(.b 

 that of typical jade ; hence we suppose ifs aflBnities to be with the jadeite-group, 



^ It is just possible that a very obscure form of this may ha\e been noticed 

 by Mr. Blake from near Cruglas, Brit. Assdc. Rep. 1888 (Bath) p. 403. 



