138 PROF. T. G. BONNEY ON A BOULDER OF HORNBLENDE 



When the Pen-y-Carnisiog rock is examined microscopically, the 

 difference between the two is rather more marked. In it the pre- 

 dominant mineral is undoubtedly hornblende. This occurs under 

 three forms : — (a) innumerable small acicular or blade-like crystals, 

 in irregular tufted groups, forming a kind of ground-mass : these 

 vary from a pale green tinge to almost colourless, and are generally 

 very feebly, if at all, dichroic ; the comparatively small extinction- 

 angle shows them to be hornblende (actinolite) ; and there can be 

 no reasonable doubt that they are of secondary origin ; (b) small 

 crystals, exhibiting often characteristic cleavages and even external 

 forms (combinations of ooP and ooP c oo), green-coloured and strongly 

 dichroic ; (c) large crystals (those mentioned above as supposed 

 augite), including grains &c. of more than one other mineral, to be 

 presently noticed. Augite occurs not unfrequently in almost 

 colourless grains and crystals, some of which show a characteristic 

 cleavage, and, in one or two cases, the characteristic outline (section 

 of the combination of ooP, ooP°oo, ooP°oo). The extinction -angle 

 of the longitudinal sections of these crystals is large, ranging com- 

 monly from 30° to 40°. They usually occur interspersed in a dull 

 olive-green serpentinous or chloritic mineral. No olivine can now be 

 recognized with certainty in the slides ; but there are a number of 

 irregular grains associated in them with the other minerals, and 

 abundantly included (these being of smaller size) in the large horn- 

 blende crystals, which there is every reason to believe are pseuclo- 

 morphs after the former mineral. Opacite and rounded crystal- 

 line grains resembling magnetite abound in these, often clustered 

 together or lining roughly parallel cracks, which remind us of the 

 irregular cleavages of olivine ; from these a brown staining often 

 extends inwards for some little distance. The pseudomorphic con- 

 stituents vary considerably : sometimes, as described above, they 

 are a brownish or yellowish green, of filmy granular structure, act- 

 ing upon polarized, light, but not greatly changing their colour; 

 sometimes they exhibit a finely-speckled aspect, as though aggre- 

 gates of extremely minute grains or folia of a mineral that acts 

 strongly upon polarized light ; sometimes they are aggregated small 

 folia of a mineral resembling talc ; and sometimes a clear isotropic, 

 or nearly isotropic, mineral, such as is common in ordinary serpen- 

 tines. Two or three of the inclusions in the larger hornblende 

 crystals exhibit a radial aggregate structure with the usual black 

 crosses. Though most of the above microlithic products are not the 

 most usual replacements of olivine, I have seen them occasionally 

 in my studies of peridotites, and have no doubt that this mineral 

 was formerly present in the Pen-y-Carnisiog rock (probably a 

 rather ferriferous variety). Magnetite is not uncommon as an in- 

 clusion in the larger hornblende crystals ; the slide also contains a 

 little altered brown mica, and a few small crystals of apatite. 



With regard to the large black crystals already mentioned, 

 although, from their optical properties, one cannot but regard them 

 as a brown hornblende, I doubt much whether this is not due to 

 subsequent paramorphic change, and believe that they were for- 



