Webster on Calton Hilt. 231 



hill, where the most distinct alternations of substances (whose 

 aqueous origin none can dispute,) with pure and well charac- 

 terized wacke are displayed, has not, as yet, I believe been 

 particularly described. 



Edinburgh is situated nearly in the centre of an extensive 

 coal formation, where the usual sandstones and other coal 

 measures are connected with the newer rocks of transition. 

 From the coal field rise in many places beds of greenstone, in 

 general forming small conical and round-backed hills. Other 

 eminences are composed of amygdaloid, claystone, and other 

 porphyries ; and basalt and trap tufif occur in an overlying po- 

 sition. Of these, it is not my intention to speak otherwise 

 than as conveying a general idea of the geological relation of 

 the wacke above referred to. 



The structure of Calton hill has been exposed by the recent 

 improvements, and in particular by a section made in the con- 

 struction of the new road to London. The rock occurring in 

 greatest abundance, and which is probably the fundamental 

 bed, is a porphyry, the basis of which in general is claystone, 

 which in many places passes into felspar, in others becomes a 

 distinct greenstone. Numerous veins of calcareous spar tra- 

 verse it in different directions, and I am lately informed, that 

 very beautiful examples of veins of greenstone of contempo- 

 raneous formation with the rock itself, have been discovered 

 in the greenstone. Upon the porphyry rests a bed of trap 

 tuff, upon this other beds of«the two rocks repose, that at the 

 summit [being porphyry. The back of the hill (as we pass 

 from the city) is a spot of peculiar interest, consisting of alter- 

 nate thin beds of bituminous shale, sandstone, wacke, and 

 clay ironstone, disposed in a manner which will be best under- 

 stood by a rough outline taken on the spot. 



