910 T. G. BONNE Y ON THE SERPENTINE AND 



sometimes forming fringes almost like tufts of grass ; they make 

 their way sometimes up cracks in the felspar, and appeal even to 

 form as cndomorphs. The difference in dichroism between these 

 and the yet unchanged diallage is well exhibited on rotating the 

 polarizer without the analyzer. I find no olivine or grains of ser- 

 pentine ; but a speck here and there resembles the latter ; the out- 

 line, however, of some of the bunches of actinolite resembles rather 

 that of olivine than diallage. As there certainly has been olivine 

 in the rock, this confirms my suspicion that this mineral sometimes, 

 instead of forming serpentine, contributes to the actinolite. When 

 the latter mineral is formed, it is remarkable how greatly the 

 cohesion of the constituents of the rock seem to have been disturbed ; 

 for it not only invades the edges of the felspar in needle-like fringes, 

 but also occurs in blades and tufts anywhere in the heart of that 

 mineral. In fact, where this process has been carried far, it is only 

 now and then that the outline of the original crystals can be traced. 



Lastly, as to the trap-dykes, I regret to say that I cannot 

 give precise information with regard to all of these, as I have 

 found it difficult in one or two cases to identify those which I had 

 observed in different visits. The notes made in 1873 and 1875 

 were taken with a rising tide ; and so, as other matters were of 

 still greater interest, 1 rather hurried over these dykes. In 1876 

 the tide was very favourable, and I observed more dykes than I had 

 noticed previously ; but still, as there was so much to do that I did 

 not collect a specimen from every one, I have a little difficulty in 

 identifying those collected on former occasions with the dykes noted 

 on this. However, the following results are fairly correct. There 

 are two varieties of rock in the dykes : — one, coarse enough to be 

 obviously a crystalline rock, much like an anamesite ; the other 

 very compact, of a dark purplish colour, looking almost like a dull 

 serpentine. Thin veins of serpentine often show in the latter ; and 

 films of the same not seldom coat joint-faces. The dyke seen in the 

 cliff may be taken as a type of the former. On microscopic exami- 

 nation it proves to be a felspar basalt, — the plagioclase being, 

 as a rule, clear, though with polarized light dull in colour, the augite 

 little changed, and dull green spots replacing probably the olivine. 

 There are also grains of magnetite, with brown stains from one of 

 the other ferric oxides ; and cracks are overspread by an infiltrated 

 serpentinous mineral. 



The second specimen (collected in 1873 from a dyke " cutting 

 gabbro and serpentine ") is quite different. It has an isotropic 

 matrix, which remains black, with crossed prisms, as the stage is 

 rotated, full of rather acicular crystals of actinolite, very pale clear 

 green with ordinary light, slightly dichroic with the polarizer, and 

 often showing brilliant colours with the analyzer. Here and there 

 are patches of the brown hornblende granulated with magnetite (?), 

 like that described from the shore north of Kynance (p. 889). Some 

 of these occur in such a way as to produce a strong impression that 

 they too have been partly replaced by actinolite. One or two 

 portions of unchanged plagioclase crystals remain in the matrix ; and 



