252 Outlines of Geology , 



begin to totter upon each other, and ultimately fall. This ten- 

 dency of square blocks to become spheroidical, and which has 

 sometimes been mistaken for the effect of friction, explains some 

 of the mysteries formerly adverted to respecting granitic boulders, 

 and shews that attrition by torrents, and transportation by streams, 

 are not always essential to their rounded appearance. The cele- 

 brated logging- stone is also noticed in Dr. Mac Culloch's com- 

 munication. It exhibits the tendency of this kind of granite to 

 cuboidal separation, and although 17 feet long, 32\ in circum- 

 ference, and weighing 65 tons, may be moved by the force of a 

 few pounds, and visibly vibrates when blown upon by a western 

 gale. 



Though granite is in general a very durable rock, and though 

 the permanence of the lofty peaks of the Alps, and other great 

 granitic chains of mountains, is such as to have enabled them to 

 weather those storms that have carried away and disintegrated 

 much of the softer materials of other, and probably of superin- 

 cumbent strata, yet there are some varieties of granite subject to 

 moulder down, and that even with no inconsiderable rapidity. 

 De Luc talks of the friable granite of the Hercynian forest, and 

 Saussure describes the mouldering down of that in the Alps. 

 The waters of the Arve are rendered milky by the pulverulent 

 felspar that comes from the Aiguilles de Chamouny, and other 

 points that border the Mer-de-Glace. The road across Dartmoor, 

 from Ashburton to Chagford, traverses in one place such loosely- 

 compacted granite as to resemble a bed of gravel. The granite 

 of the Carglaise mine near St. Austle, in Cornwall, is so soft and 

 pulverulent, that the excavation might be mistaken for a chalk- 

 pit ; and in the same vicinity, the immense quantities of white 

 porcelain-earth, as it is called, is of similar origin, and seems 

 derived from the perishable nature of the felspar, which, giving 

 way, suffers the quartz and mica to fall out. To what the ex- 

 treme proneness of some kinds of granite to suffer decay, while 

 others are as remarkably permanent, is to be attributed, does not 

 seem quite clear ; but if I mistake not, Sir H. Davy, in his geolo- 



