358 LIEUT.-GEN. C. A. M c MAHON ON ROCKS OF [Aug. 1 894, 



augite was most abundant. The slice depicted in fig. 2 consists of 

 felspathic base and augite ; it contains very little hornblende, and 

 the shading given by the artist is somewhat misleading in this 

 respect. 



Assuming that these rocks were originally ashes, it is evident 

 that they must have suffered considerable metamorphism since they 

 were laid down. There is no difficulty in accounting for this : 

 the rocks under description occur within a few yards of the Cock's 

 Tor epidiorite, and the epidiorites of this area are believed by 

 previous writers to have exercised a metamorphic influence on the 

 strata adjoining them. 1 Then the altered ash-beds here described 

 are less than | mile from the main mass of the Dartmoor granite, and 

 in its underground extension the granite may be even nearer than 

 this. A subterranean connexion between the granite of Dartmoor 

 and Brown Willy has been considered probable by those who have 

 already written on the subject, from De la Beche to Ussher. 2 The 

 rocks under consideration lie near the axis of this supposed under- 

 ground extension of the granite, and the potency of the contact- 

 ractamorphism exercised by the Dartmoor granite has been admitted 

 by numerous observers. 3 



As the massive dolerites, now epidiorites, have suffered so many 

 mineral changes from the presence of the granite, it does not make 

 a severe demand on our faith to believe that beds of finely triturated 

 volcanic dust also felt the effects of the contact-action of the great 

 mass of the Dartmoor granite, and of its underground extension 

 between Dartmoor and Brown Willy. 



These Cock's Tor beds appear to have been sufficiently distant 

 from the nearest crater to have escaped the shower of coarse 

 materials, and to have received only the fine dust borne upon the 

 wind. No one who is acquainted with the history of Pompeii and 

 Herculaneum, and has seen the deep and fine-grained deposits 

 under which those cities are buried, need hesitate in believing 

 that beds of equally fine-grained volcanic material, devoid of large 

 fragments, may have been deposited at Cock's Tor. If so, I see 

 no difficulty in going a step farther, and supposing that when 

 the finely triturated particles of highly basic lava, charged with 

 water, were subjected to the long-sustained heat and pressure 

 which must have resulted from the intrusion of the great mass of 

 Dartmoor granite, a reconstruction of the materials took place, and 

 pyroxene and felspar were formed. 



1 I express no opinion on this point, myself. The subsequent metamorphism 

 by the granite renders the testing of this conclusion very difficult. 



2 W. A. E. Ussher, ' British Culm Measures,' Proc. Somerset Archseol. & 

 Nat. Hist. Soc. vol. xxxviii. (1892) p. 193, where De la Beche is quoted. 



3 Ibid. p. 209 ; see also Teall, ' Brit. Petrography,' p. 234 ; Worth, ' Eocks 

 of Plymouth,' Trans. Plym. Inst. vol. ix. (1886) pp. 242-246 ; De la Beche, 

 ' Report on the Geology of Cornwall, Devon, and West Somerset,' 1839, p. 267 ; 

 Allport. Quart. Journ. Geol. Soc. vol. xxxii. (1876) p. 421 • Rutley, Geol. Surv. 

 Mem. ' Brent Tor,' p. 25 ; and the writer's previous paper, Quart. Journ. Geol. 

 Soc. vol. xlix. (1893) p. 389. 



