NEAR THE WESTERN COAST OF ANGLESEY. 255 



I have examined microscopically specimens from four of the above 

 localities. Numbers III. and IV. are, macroscopically, almost undis- 

 tinguishable from the Pen-y-Carnisiog specimens ; nor is there any 

 material difference under the microscope. The larger hornblende cry- 

 stals in III. are commonly a light brown, but occasionally a pale green. 

 The smaller crystals are more commonly green. Small portions of 

 some of the larger crystals, sometimes external, sometimes internal, 

 are almost colourless, and have a rather more milky aspect than the 

 rest of the crystal. With crossed nicols they exhibit a different 

 tint, but extinguish either at or as nearly as possible at the same 

 angle. Still, even then, on introducing a quartz plate, a marked 

 difference of tint is perceptible. Whether this change denotes an 

 hydration of the hornblende or a paragenesis of two slightly dif- 

 ferent varieties, I cannot say. Cracks in the crystals are often filled, 

 and the edges bordered, with a minutely granular mineral, giving 

 light specks of colour with crossing nicols. In parts of the slide 

 are many small grains and clusters of a mineral now consisting of 

 aggregates of this secondary mineral and of earthy-looking dust. 

 These may possibly have been a rather aluminous augite; this 

 mineral, however, as in the case of the Pen-y-Carnisiog rock, cannot 

 be certainly identified in the slide. There are numerous grains of 

 a dark brown mineral, in some cases feebly translucent, in others 

 including granules of a clear light-brown mineral ; a few appear to 

 be sections of octahedra. They present a resemblance to chromite ; 

 but, as no chromium has been detected on analysis, this mineral can 

 hardly be present ; possibly they are spinel. For the various pseudo- 

 morphic products occupying the rounded grains in the larger crystals, 

 and the general ground-mass of the slide, I may refer to my former 

 paper. I do not identify mica. It is just possible that the slide 

 may have contained a crystal or two of felspar. 



jSTo. IV. differs so little from the last tbat a separate description 

 is needless ; there can, I think, be no doubt that some of the serpen- 

 tinous aggregates replace olivine. One exhibits an approach to 

 aggregate polarization. There is a little apatite, and a few scales 

 of brown mica. 



]S"o. II. presents, macroscopically, some slight varietal differences : 

 the ground-mass is greener ; and the imbedded hornblende crystals 

 are not quite so abundant or large as in the other cases, being 

 commonly from 0*2 to 0*3 inch in diameter. But the microscopic 

 structure has a close general resemblance to that above described, 

 though it contains a few grains of a serpentiuous mineral which I 

 have not observed in the others. This has a rather irregular outline, 

 is nearly colourless, is of a somewhat silky structure, with rather in- 

 frequent cleavage-planes parallel with the fibres of this structure ; 

 it extinguishes when they coincide with the vibration-plane of either 

 of the crossed nicols. Numerous minute belonites, slightly browner 

 in colour, occur in the grains, lying often so as to cross one another 

 at angles of about 120°, and to be very nearly bisected by the cleavage- 

 planes. I have already noticed this microlithic structure, sometimes 

 w T ith minute rods of opacite, in one of the minerals of various serpen- 



