The country around Edinburgh is extremely interesting to the geologist, and presents numerous instances of the junction of rocks to which the advocates of the Neptunian system have referred in support of their opinion as to the aqueous origin of greenstone, basalt, and wacke; while the same examples have been cited by the Volcanists, and by those who hold an intermediate opinion. The structure of a portion of Calton hill, where the most distinct alternations of substances (whose aqueous origin none can dispute,) with pure and well characterized 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 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 construction 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 beautiful examples 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 will be best understood by a rough outline taken on the spot. [Illustration: Monument to NELSON. 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 11 Wacke--and _calc. spar._ 12 Bituminous shale. 13 Wacke. 14 Bituminous shale. 15 Wacke. 16 Bituminous shale. 17 Sandstone. ]