580 PEOE. T. G. BONNET AND MISS C. A. EAISIN ON [Nov. 1 894, 



of the strongest and clearest nature, if it is to establish the supposed 

 unconformity. This evidence, in all places but one, is indirect ; we 

 ■will consider that instance first, and then pass on to the other 

 cases. 



II. Evidence eeom Moel Tevfaen. 



This locality is the one which seems at first sight most to support 

 Mr. Blake's hypothesis. As we were unable to spare the time for 

 any thorough re-investigation of the adit, 1 we have considered how 

 far the surface-exposures, taken in conjunction with Mr. Blake's 

 description of the tunnel, 2 necessitate the acceptance of his hypo- 

 thesis. The general succession crossed on the western slope 

 above and north of the Bryngwyn incline (that described as ' the 

 southern slopes') is claimed as representing the tunnel-section, 

 but it is not very clear how the author accounts for the absence 

 from the tunnel of the conglomerate of the tramway-cutting. 3 This 

 is at a lower level and strikes towards the adit, which, it is admitted, 

 commences in felsite. 



The main difficulty, however, is the apparent absence of the 

 summit-conglomerate from the tunnel. Certainly nothing in the 

 detailed section given by Mr. Blake can fully represent the extensive 

 outcrop on the top of the hill. Assuming the accuracy of his obser- 

 vations of the tunnel, there seems on our theory no other explana- 

 tion possible than that this conglomerate is faulted out ; and the 

 broad outcrop at the summit might be due partly to such dis- 

 turbance. This seems suggested by the changed dip found in the 

 associated green grits. 4 



The chief arguments drawn by Mr. Blake from the surface ex- 

 posures are, that the dip is nearly horizontal, and that the northern 

 slopes are covered by conglomerate and grit. Special reference is 

 made to two lines of crags in this direction. It is said that the 

 upper " most distinctly show a low dip of not more than 5° to the 

 east" (op. cit. p. 462). We took the dip on several blocks and sur- 

 faces, of which four at least were clearly shown varying from 15° to 

 25° generally to S. of E. or S.E. 6 ; while in the grit of the summit- 



1 Each of us on separate occasions has been through it, but not with good 

 lights. 



2 That difference of opinion is possible would appear from the fact that a 

 specimen which one of us collected at a hundred paces from the mouth or 

 northern end of the adit, that is, somewhere in ' 29, light crystalline felsite,' is 

 an unquestionable felsitic grit. 



3 Prof. Bonney in 1878 collected a specimen from the spoil-bank at the N.W. 

 mouth of the adit which closely resembles this conglomerate, and noted 

 ' Cambrian conglomerate ' as one of the four varieties of rock lying about. In 

 1880 he passed through the adit (but only with a bull's-eye lantern) and found 

 ' Cambrian conglomerate ' following after the felsite with (?) some felsitic 

 grit. This is probably identical with the rock mentioned by Mr. Blake in a 

 note (op. cit. p. 460). 



4 These do occur, although Mr. Blake says (op. cit. p. 462) : — ' nor can we find 

 any green grit on the summit.' 



These, in some cases, are the dips shown on exposed surfaces, and so may 

 be less than the true angle. 



