1#19.] and on the Aqueous Origin of Wacke. 255 



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 tuff occur in an overlying position. 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 traverse it in 

 different directions ; and I am lately informed, that very beauti- 

 ful examples of veins of greenstone of contemporaneous forma- 

 tion with the rock itself, have been discovered i«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 ofpecu- 

 liar interest, consisting of alternate thin beds of bituminous shale, 

 sandstone, wacke, and clay ironstone, disposed in a manner 

 which will be best understood by a rough outline taken on the 

 spot. 



A. Porphyry. 



B. Trap tuff. 



C. Porphyry. 



D. Trap tuff. 



E. Porphyry. 



F. Beds of wacke, &c. upper 

 part concealed by vegetation. 



1. Bituminous shale. 



2. Wacke. 



3. Sandstone. 



4. Bituminous shale, with clay 

 ironstone. 



5. Wacke. 



6. Bituminous shale 



7. Wacke, with calc. spar. 



8. Bituminous shale. 



9. Wacke. 



10. Bituminous shale passing 

 on both sides into 



1 1 . Wacke, and calc. spar. 



