782 T. G. BONNET ON THE SERPENTINE AND 



tints with crossed prisms. The viridite has a finely fibrous struc- 

 ture, and sometimes a banded arrangement. It is fairly dichroic, 

 exhibits aggregate polarization, and shows low tints, varying from 

 pale greenish blue to dull brown, as the polarizer is rotated. It is 

 doubtless allied to delessite*. I have seen it occasionally in basalts, 

 e. g., in one from Hawkscrag, Pifeshire coast, where it replaces 

 augite. Here it is more doubtful what the original mineral has 

 been, since the augite which remains seems so well preserved f. 

 Either it has been olivine (but I hardly think so) or a different 

 variety of augite. A few globulitic grains of granular structure 

 and semiopaque brown tint are scattered in it. From the micro- 

 scopic examination I conclude that this rock is a rather more basic 

 porphyrite than the other one. 



It too appears to rest on a tuff (already described), and I had 

 suspicions that the latter was unconformable with the supposed 

 Silurian rocks, though I could not obtain evidence on this point 

 which was quite conclusive. The porphyrite extends along the 

 coast for nearly a mile, and is here and there cut by a basalt dyke. 

 Seeing that the older rocks of the district appear to be inclined at a 

 high angle, we might well be surprised, if it were one of that series, 

 at the great thickness and homogeneous character of this rock. To 

 my mind the extent of ground covered by it is strongly in favour 

 of the idea of this porphyrite being part of a lava-stream that has 

 passed over the denuded edges of older beds. 



Later Basaltic DyTces. 



There are occasional rather narrow dykes of basalt cutting the 

 above-described porphyrites, as well as that mentioned above (p. 780.) 

 At these, as a rule, I only glanced, as there was nothing in them, so far 

 as I saw, different from the dykes so abundant in Western Scotland. 

 I took, however, a specimen from one, that marked on the map just 

 to the north of Balcreuchan Port, which I have since examined. It is 

 a compact green-black basalt with many rounded white spots, like 

 minute amygdaloids, showing on fractured surfaces a rather fibrous 

 structure and slightly silky lustre. Under the microscope both the 

 plagioclase and augite of the rock are seen to be much altered. The 

 white spots show an isotropic mineral which is transparent, except 

 where somewhat clouded with fine dust, and has an imperfect 

 cleavage. It may be analcime. In it, generally about the edges, 

 are a great number of minute rosettes of a greenish mineral, fairly 

 dichroic, and showing a pale golden colour and an aggregate polari- 

 zation. Part of one of these amygdaloids is almost wholly occupied 

 by a different mineral. It crystallizes in radiating needles, belonging 

 apparently to the orthorhombic system, with a fairly marked basal 

 cleavage. "With crossed Mcols it shows rather bright colours. It may 

 possibly be stilbite. 



Near Lendalfoot hamlet, however, are some dykes which appear to 



* Zirkel, Geol. Explor. &c. p. 13. 



t The augite and viridite are sometimes contiguous with a perfectly sharp 

 line of demarcation. 



