208 DOLOMITIC CONGLOMERATE OP TiEISTOL. 



mcntionod railway tlirongli Alraondsbury Ridgo. Tlio 

 distance apart of the old and new tunnels, from centre to 

 centre, is not much over forty feet, and between their sides 

 not more than twenty feet; and yet during tlio construction 

 of the first tunnel, the Conglomerate was not found, or at 

 any rate noticed to bo there, nor is it shown on the geologi- 

 cal section taken during its construction. It seems hardly 

 likely that it would thin out or suddenly disappear in such 

 a short distance ; and yet such has been the case, or, at any 

 rate, to a very great extent. It is quite possible that a 

 small portion may have boon passed through unnoticed in 

 constructing the tunnel, and may have been omitted from 

 the section. The Conglomerate found, however, in the new 

 tunnel is sixteen foot in thickness, and was amongst the 

 hardest of the rooks found in the tunnel, and gave the most 

 trouble. It is certainly most strange, therefore, that it 

 should have been passed through unnoticed. The top of 

 the Conglomerate found in the now tunnel runs nearly 

 level for about two hundre<l j^ards, from whence it suddenly 

 rises at nearly as great a dip as the Carboniferous rocks, 

 over the edges of which it passes. The ordy explanation 

 that appears probable of the absence of this Conglomerate 

 in the old tunnel, the bottom of which is about on a level 

 with the crown of the new tunnel, is, that when it makes its 

 sudden rise over the edges of the older rocks it comes to a 

 sudden end, and therefore did not rise up to the level of the 

 old tunnel, or at any rate was only met with slightly in its 

 bottom, and therefore remained undistinguishod from the 

 following beds of hard grit. It consisted chiefly of particles 

 of Millstone Grit, some of them of considerable size and 

 exceeding that of a man's head, together with pieces of 

 Carboniferous Limestone, but chiefly of a mxich smaller size. 

 Towards its j'unction with the Carboniferous rocks it was 



