Webster on Calton Hill. 931 
hill, where the most distinct alternations of substances (whose 
aqueous origin none can dispute,) with pure and well character- 
ized wacke are displayed, has not, as yet, I believe been partic- 
ularly described. 
inburgh 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 smali conical and round backed hills. Other eminences 
are Composed of amygdaloid, claystone, and other porphyries; 
and basalt and trap tuff occar in an overlying position. Of these, 
itis not my intention to speak otherwise than as conveying a 
Seneral idea of the geological relation of the wacke above re- 
ferred to, : 
The structure of Calton bill has been exposed by the recent 
improvements, and in particular by a section made in the construc- 
tion of the new road to London. The rock occurring in greatest 
abundance, and which is probably the fundamental bed, is a por- 
Phyry, the basis of which in general is claystone, which in ma- 
ny places passes into felspar, in others becomes a distinct green- 
‘tone. Numerous veins of calcareous spar traverse it in differ- 
eat directions, and I am lately informed, that very beautiful ex- 
8 of veins of greenstone of contemporaneous 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 alternate thin beds of bituminous shale, 
Sandstone, wacke, and clay ironstone, disposed.in a manner which 
Vill be best understood by a rough outline taken on the spot. 
