﻿[ 560 ] 



below the rocky crest due 

 north of Tan-y-graig (see 

 fig. 4). Here a mass of ash, 

 about 81 feet long and of 

 variable thickness, lies in the 

 slates some little distance 

 below the main ash-band. 

 At its western end it termi- 

 nates somewhat abruptly, 

 but at its eastern end it 

 tails out gradually. Where 

 it attains its maximum 

 thickness it seems to be in 

 actual contact with the main 

 ash-band above. Owing, 

 however, to the difficulty of 

 examination at this point, 

 we cannot be certain that it 

 is not separated by a narrow 

 band of slate. At the 

 western extremity of the 

 mass a favourably - placed 

 crag shows a transverse sec- 

 tion of the band, and there 

 is here a complex inter- 

 penetration of ash and slate 

 curiously suggestive of an 

 intrusive junction ; but it 

 suggests that the slate has 

 been squeezed into the ash, 

 rather than that the ash 

 has been intruded into the 

 slate (see fig. 5, p. 561). 



Moreover, in this neigh- 

 bourhood the relations of 

 the slates to the interbedded 

 sandstone - bands are very 

 similar. A few feet below 

 the mass of ash just de- 

 scribed, several blocks and 

 short bands of sandstone lie 

 within the slates. They are 

 not lenticular in shape, but 

 often terminate in abruptly- 

 squared or rounded ends ; 

 and in many cases they are 

 evidently portions of bands 

 that were once continuous. 

 These bands have since been 

 broken up, and the slate 

 has been squeezed between 

 the separated blocks. 



