Outlines of Geology. 253 



gical lectures delivered in this Institution, considered it to arise 

 from the alkaline matter of the felspar being predominant, and 

 yielding to the solvent agency of water. 



Independent, however, of chemical composition, mere mecha- 

 nical texture, and the general aggregation of mountain masses, 

 has much to do with their respective durabilities. Where the 

 arrangement of granite resembles that prevalent in the greater 

 part of Cornwall, water gradually penetrating between the blocks 

 and masses, freezes there, and thus slowly removes them, or trans- 

 fers them to unstable ground : while the firmer lamellar texture 

 of much of the Scotch, Alpine, and other varieties of granite, 

 denying access of water to its fissures, is slower in suffering the 

 decay that is referable to that very powerful cause. 



Gneiss and mica-slate often form mountain masses in associa-* 

 tion with each other, and with the varieties of granite. The 

 former is seen singularly contorted upon the coast of Lewis ; 

 and mica- slate rock is associated with the serpentine of Corn- 

 wall, and is seen in great perfection among the Scotch gra- 

 nitic scenery, more especially in the vicinity of Dunkeld, and 

 in extraordinary magnificence in the lofty mountain of Ben- 

 more, the summits of which are above 4,000 feet above the 

 level of the sea, and when seen in certain directions in respect to 

 reflected light, dazzle the spectator by their extent and brilliancy. 

 Ben Lawers, on the north of Loch Tay, as well as many of the 

 neighbouring mountains, furnish the geological student with 

 highly-instructive specimens of granite passing into gneiss, and 

 this again degenerating into mica-slate and chlorite-slate. I know 

 of no school of geology superior to this district ; the transitions, 

 junctions, and varieties of rocks are without number, and in many 

 places so near each other, as readily to come within the eyes* 

 grasp, and so accessible, as to furnish even the timid climbe* 

 with a fine and useful series of specimens. To mention one, among 

 the many places of this description, there is a quarry upon the 

 high road, about three miles south of Dunkeld, in which a stratum 

 of grauwacke is seen incumbent upon chlorite-slate, gradually 

 passing into a fine gray roofing-slate, and this recumbent upon 



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