896 T. G. BONNEY ON THE SERPENTINE AND 



the planes of principal cleavage ; until at last the whole crystal is 

 converted into an aggregate of small crystals of hornblende. From 

 the general appearance of the mineral I take it to be actinolite, 

 which as a non-aluminous variety of hornblende would most 

 readily be formed from ordinary diallage. In many fcases it is 

 almost fibrous in structure ; then it is paler in colour and feebly 

 dichroic. The specimens which I have examined have not shown me 

 any olivine ; yet, as we shall presently see, this mineral abounds in 

 thegabbro further north. This absence seems strange : I have, how- 

 ever, some reason to think that it, too, in this case, has been replaced 

 by actinolite*. 



The " granite vein " headland is a prominence on a rather larger 

 one ; beyond this is a little chine and another small headland. Over 

 this space serpentine predominates ; but fragments of hornblende 

 schist are included, and intrusive dykes and veins of gabbro are 

 common. Every step shows something new and interesting. At one 

 place the gabbro on the left, and the serpentine on the right, make 

 an almost vertical junction in the cliff. The former includes a long 

 strip of hornblende schist in an upright position ; the latter assumes 

 near the junction a rather fissile character. Often it would be 

 hard to say whether the serpentine or the gabbro were the intruder ; 

 but here and there may be found conclusive evidence that the latter 

 is the newer. The gabbro is coarse, the diallage crystals being some- 

 times very large, one composite specimen being about 6" X 2" x 2". 

 The foliated structure mentioned above is often seen ; and I observed 

 that, as a rule, it was best developed where the gabbro intersected 

 the hornblende schist, especially where it had passed between two 

 masses along the plane of bedding. As the whole mass cooled, these 

 already solid schists would doubtless produce a definite pressure on 

 the crystallizing rock between them at right angles to their bounding 

 surfaces, and so determine its structure. I have seen the mica 

 crystals in a granite vein which had broken through angular frag- 

 ments of a schist lying at right angles to normals from their surfaces, 

 and have often observed that on the outside of a granite vein the 

 mica plates tend to lie parallel to the surface. 



The next headland exhibits both serpentine and gabbro intrusive 

 in schist, with a large felspar vein. But it is needless to carry these 

 details further ; so I will select one more section for description, the 

 last which can be reached from the shore. Here a lofty cliff of 

 serpentine is shattered by veins of gabbro, one of which, about ten 



* I have good reason to believe that this replacement of augite or diallage 

 by a form of hornblende has taken place in several of the Welsh "green- 

 stones." It is not precisely a paramorphic process like the formation of 

 uralite, nor a pseudomorphic, because the form of the original crystal is often 

 lost, but a replacement of a mineral by another, which, if not really a dimorphous 

 form of the first, is very closely allied to it. I conceive the change to have 

 been mainly brought about in the " wet way." I have seen the same change in 

 the gabbros from Mont Colon (mentioned above), and the Matterhorn, 

 also in the hypersthenite of Penig (Saxony). MM. Poussin and Eenard have 

 observed it in the Ardennes rocks (Roches Plutoniennes de la Belgique, &c. 

 P . 69). 



