170 Geological Observations made in Scotland, 



same bright grey, rough, and corroded, superficial crust, 

 produced by weathering, which is so striking in the porphyry 

 of Glencoe. There appears also on its outer surface a brec- 

 ciated structure, which gives it a deceptive resemblance to 

 a sedimentary rock ; although, when freshly quarried, it has 

 the appearance of a homogeneous mass. Greater still is the 

 similarity of the red rocks of both localities. The granite of 

 Ben-Nevis passes into a red porphyry so like that of Glencoe, 

 that I could not distinguish the one from the other. In 

 Glencoe, likewise, the intimate connexion of the granites 

 in the bottom of the valley, and in the vicinity of Kings- 

 house, with the porphyry which rises in dikes, can scarcely 

 be doubted. This constant association of trap-like por- 

 phyries, — ^black, and poor in quartz, — with red granites and 

 porphyries, is a remarkable fact. We find this associa- 

 tion not merely in Scotland, for it occurs also in Thurin- 

 gia, in the Palatinate, in Provence, and in the long zone 

 of porphyritic hills which run along the southern border of 

 the Alps, from Piemont to Stiermark. How vividly does 

 Ben-Nevis remind one of Monte Mulatto, at the mouth of 

 the Fassa Valley, where also red granite forms the basis, 

 and black porphyry the upper mass of the mountain ! — Wil- 

 lingly would I have investigated these relations farther, but 

 the important trap islands of the western coast lay before 

 us ; and although steamboats and railways now afford great 

 facilities, yet, with the deduction of Sundays and rainy days, 

 we had already but too little time remaining to devote to 

 these fortresses of our science. With regard to the facilities 

 for travelling to which I have just adverted, I may mention, 

 that on one beautiful day, we proceeded from Fort- William 

 round the Isle of Mull to Oban, having stopped on the way 

 for some time at Stafik and lona ; and on another day we 

 went from Arran to Edinburgh. The flying view which we 

 thus obtained, served to strengthen the conviction that, for 

 the development and completion of the detail of the new doc- 

 trines respecting granite and trap, a more favourable ground 

 could scarcely be obtained than that which is aff^orded by 

 Scotland and its islands. 



In this country one is incited also, in a peculiar degree, to 



