118 Mr Lyell on a Dike of Serpentine cutting through 



side of the river, some angular blocks of greenstone are strew- 

 ed over part of the space where the dolomitic serpentine has 

 decomposed, from which we may presume that greenstone is 

 there associated with the serpentine, of which connection, un- 

 fortunately from the want of a section, no distinct knowledge 

 can be obtained. 



To return to the left bank of the river. The dolomitic 

 serpentine is flanked by a vertical mass of sandstone conglo- 

 merate, evidently much altered, about five yards thick. Some 

 parts of this rock approach to jasper in fracture and appear- 

 ance. A l-emarkable fact was pointed out to me by Professor 

 Buckland with regard to this conglomerate. The quartzose 

 pebbles which it incloses are split, and sometimes firmly re- 

 united at an angle by ferruginous matter which abounds in 

 the stratum. The cracks sometimes divide the pebbles into 

 two or more fragments, since cemented together again ; some- 

 times they only penetrate a short way in. 



This phenomenon is the more worthy of notice, as it also 

 presents itself in the conglomerate which flanks a large dike of 

 greenstone on the Isla in this county. 



That the conglomerate, however, on the Carity has, in this 

 instance, been altered by the neighbourhood of the dolomitic 

 serpentine, cannot be with certainty affirmed, since the rock, 

 which has been in contact with it on the eastern side, has de- 

 composed, and if greenstone has existed there, the case would 

 only be a parallel one with that above alluded to on the Isla. 

 I have no reason, however, to conclude that there has been 

 greenstone there, for, as far as can be presumed from the op- 

 posite bank, it appears to be sandstone and shale which has 

 wasted away, and left an interval of forty yards or more. 



Strata of sandstone, shale, and conglomerate, when harden- 

 ed by their vicinity to trap, generally resist decomposition more 

 than either greenstone or unaltered sandstone. The last rock 

 which I shall mention as forming part of the above section 

 exposed on the banks of the Carity, is a trap dike from thirty 

 to forty yards wide, which has almost entirely decomposed, 

 and left an interval of argillaceous soil. This dike is parallel 

 to the great dike of serpentine. 



When it can be seen, it is composed on the right bank of a 



