L, A. Necker de Saussure Voyage en Ecosse. 175 



hills, but they are disposed in scattered fragments, and in so- 

 litary conical hills, which rise here and there out of the bosom 

 of the plains. Sometimes, on the top of a mountain composed 

 of rocks of a different formation, there starts up a ridge of trap 

 rocks, vertically like an enormous wall. Lastly, the character 

 which distinguishes them from the greater part of known rocks, 

 is their property of causing singular modifications and remark- 

 able changes on the rocks on which they repose, or which they 

 penetrate in the form of veins. This truth, hitherto little attended 

 to, has been insisted on only by Dr. Hutton and his pupils. 



The greater part of the hills round Edinburgh belong to the 

 trap formation. Arthur-seat, the Calton-hill, the rock on 

 which the castle is built, Braid and Blackford hills, and a part 

 of the Salisbury-crags, are trap-rocks ; as well as several masses 

 on the northern bank of the Firth of Forth, such as the rocks 

 of North Queensferry, of Burntisland, of Kinghorn, and Kin- 

 caid near Ely. On the southern shore of the gulf we have also 

 the hill named Taprene Law, above Haddington ; the rock of 

 the Bass; North Berwick Law; the islands of Faidra ; Craig- 

 leith, and the rocks of Dunbar. The greater part of the 

 Pentland-hills, also, probably belongs to the same formation. 



The great precipitous rock on which the castle of Edinburgh 

 is built, consists entirely of a beautiful black basalt, whose 

 surface exhibits, from decomposition, a whitish or yellowish 

 crust. Here we have neither beds nor prismatic columns, but 

 a mass divided by numerous fissures into irregularly angular 

 pieces. On a comparison of specimens, Mr. Necker hnds that 

 the basalt of the columns at the base of the Puy de Dome, in 

 Auvergne, on the road between this mountain and Clermont 

 Ferrand, is precisely the same as that of Edinburgh, and that 

 the basaltic mole of Mont-Fernier, near Montpellier, differs 

 from the above only by the argillaceous odour it exhales when 

 breathed upon. In the western part of the Castle-hill, at the 

 bottom of the basaltic rock, the horizontal beds of sandstone, 

 which support it, have been discovered. This sandstone is 

 white, singularly hard and compact, and belongs to the coal 

 formation that forms the elevated ground on which the Old and 

 New Towns of Edinburgh stand. A perfectly distinct line of 

 demarkation separates at the above point, the black basalt of 

 the trap formation from the white freestone. There is no gra- 

 dual transition between these two rocks so different in their 

 nature, origin, and antiquity. 



A green porphyry, of an earthy consistence, covering a con- 

 glomerate, forms the greater part of the Calton-hill, that pic- 

 turesque eminence, which every visiter remembers with delight. 

 The base of the porphyry is a wacke of a dark-green colour, 

 having a dull, earthy fracture, and affording, when breathed 

 on, a strong argillaceous smell. This is the paste in which are 



