234 



position it resembles the former. A rock from Ernsettle may be 

 described as an ophitic mica-bearing diabase containing much apatite. 

 The abundance of the apatite in these rocks is often a striking feature. 

 Deeply coloured augites are comparatively rare in the basic rocks of the 

 West of England. They occur, however, in the " Bottor Eock " near Bovey 

 Tracy, This rock contains biotite together with much iron-ore and 

 apatite, The hornblende- and biotite-bearing diabases of the West of 

 England have affinities with the hornblende-diabases of North Wales 

 described by Mr. TAWNEY (see Fig. 1, Plate VIII.). It is interesting to 

 note that both in North Wales and the West of England we have 

 normal ophitic diabases and hornblende-diabases and that in both 

 districts the former rocks are much more extensively developed thin 

 the latter. 



We have now to consider the metamorphic phenomena described 

 by ALLPORT and PHILLIPS. These are identical with the phenomena 

 described by LossEN/ 1 ) LiEBE, (2 > SHENCK,^ and RENARD < 4 > as occurring in 

 the Hartz, Thuringia, Westphalia and the Ardennes ; that is in the other 

 portions of the old mountain axis of northern Europe. 



It has already been mentioned that the metamorphism is more marked 

 in the west than in the east. In Devon and eastern Cornwall we frequently 

 find rocks which retain many of the characters they possessed at the time 

 of consolidation. In western Cornwall, on the other hand, it is rare 

 to find rocks in which these characters have not been to a great extent 

 destroyed. In many cases the chemical composition is almost the only 

 original character which the rocks have retained ; and, in some cases 

 there is reason to believe that even this has been considerably modi- 

 fied. In mineralogical 'composition and texture many of these rocks 

 are far removed from normal igneous products and closely allied to the 

 crystalline schists. In the neighbourhood of the granite-masses, especi- 

 ally those of Dartmoor and the Land's End, we find highly meta- 

 morphosed dolerites. These have been described by Messrs. ALLPORT, 

 PHILLIPS, and RUTLEY. Precisely similar rocks have been described by 

 LOSSEN from the contact zone of the Rammberg mass of granite. The 

 examination of a large series of sections is absolutely essential to the 

 correct appreciation of the character of the metamorphism. Various 

 stages of metamorphism may frequently be observed in one and the same 

 rock-mass. The most striking feature is the replacement of augite by 

 hornblende. Every gradation may be observed from a rock extremely 

 rich in augite to one in which no trace of augite remains. The dis- 

 appearance of the augite is invariably accompanied by an increase in the 

 amount of hornblende or chlorite, generally the former. The hornblende 



(1) Jahr. d. k. preuss. geol. Landesanstalt fur 1883, und 1884. 



(2) Ubersicht iiber den Schichtenaufbau Ostthiiringens. Abh, z. geol. Specialkarte von 

 Preussen, &c. Berlin, 1884. 



(3) Die Diabase des oberen Ruhrthals, Inaug. Diss. ' Bonn, 1884. 



(4) La Diabase de Challes. Bull. Acad. Koy. Belg. Vol. XLVI., 1878, 



