198 Analysis of Scientific Books. 



moreover, iu the order in which we have named them, re- 

 present pretty nearly the geognostic position of these masses, 

 in reference to their antiquity ; for the trap-formation, pene- 

 trating the secondary formations in veins, is more recent than 

 them ; whilst the granitic and porphyritic formations are them- 

 selves traversed by trap veins, and do not penetrate beyond the 

 transition beds. Lastly, the veins of porphyry traverse the 

 granitic masses, which shews that the latter are the most 

 ancient of the particular formations. 



Numerous veins of granite traverse the gneiss, between the 

 Bay of Laxfiord and Cape Wrath. The Ord of Caithness, a 

 mountain 1,200 feet high, is a species of graphic granite, rising 

 out of the gneiss district. But it is in the mica-schist for- 

 mation that the most considerable granitic masses are to be 

 found. Thus the district of Braemar, bounded to the north- 

 east, by the valley of the Spey, to the south-east by Glen Tilt, 

 forms a great granitic island, amidst the mica-schist and its 

 subordinate rocks, across which veins of granite extend to a 

 great distance. Other similar groups, although less extensive, 

 are seen in the county of Aberdeen, between the rivers Dee and 

 Don, and on the banks of the Linnfe-Loch and Loch-Leven 

 in Invernesshire. The prevailing mass of this last group is 

 a sienitic granite. Near Loch-Rannoch there is a mass of 

 granite, eight miles long ; and the southern foot of the Co- 

 riarich mountain is sienitic granite. Masses and veins of 

 granite are found on the borders of Loch-Ness. The granite of 

 the island of Arran rises evidently in the middle of moun- 

 tains belonging to the chlorile-slate formation. But what is 

 most remarkable, is to find masses of sienitic granite, which 

 pierce their way across beds of the greywacke formation, as is 

 observed in the county of Kircudbright, in the south of Scotland. 

 Three districts, or islands, separated from each other by beds 

 of greywacke, are occupied by granite-formations. The first 

 mass is on the banks of Loch-Doon, the second on those of 

 Loch-Ken, and the third on the sea-shore, between the mouth 

 of the rivers Nith and Urr. Here Mount CrifFel, a granitic 

 mass, is nearly 1,700 feet high. 



Ben-Nevis and Ben-Cruachan, the two greatest mountain 

 masses in the island, are composed of porphyry. 



The rocks of the trap-formation, relative to their mineralogicai 

 composition, may be divided into three classes. The first 

 is the trap-greenstone, of which we have an example in 

 the rock of Salisbury-craig; 2rf, The basalt, like that of 

 Arthur-seat, and of Edinburgh-Castle; and lastly, the trap- 

 porphyries and feldspar rocks, such as the porphyries, pitch- 

 stones and obsidians of Arran and Eigg. The two banks of 

 the Firth of Forth up to Stirling, are characterized by the 

 abundance of greenstones and their wackes. In the district 



