JONES — ON THE WEATHERING OP GRANITE 



307 



weaker, action of tliese processes we see already producing effect on 

 the exposed gTanite-faces of tlie old quarry at its foot (fig. 6) . 



Granite (like otker rocks of igneous origin, and like many of 

 aqueous formation also) is always cut tki^ougli by joints or fissures 

 more or less regTdarly, and tliese have probably originated in tlie 

 contraction of the rocky mass whilst cooling. The rain-water trickles 

 through these lines of joints, decomposes the granite along the 

 cracks, widening them and rounding [off the angles of their inter- 

 sections, and ultimately only the harder masses, or the hearts of the 

 blocks defined by the joints, remain as solid crystalline granite, some, 

 though little, of the quartz of the gTanite is dissolved away by the 

 water ; the iron becomes oxydized and weakens the rock ; but it is 

 chiefly the felspar that is decomposed by the action of carbonic acid, 

 its alkalies are removed, and its residue is w^ashed away in the form 

 of white clay, the material so useful to porcelain-manufacturers, and 

 prepared artificially to a large extent from felspar-rocks. The 

 quartz-crystals remain as sand ; the mica also remains, but is less 

 observable, and is partially decomposed. 



Prof. J. Phillips has the following pertinent remarks on the waste 

 of felspathic rocks (Manual of Geology, new edit., 1855, p. 488) : — 



" The exterior of most uncrystalline rocks and buildings seem to 

 be slowly eaten away by the moistm'e and carbonic acid of the air ; 

 but the influence of this destructive agent is most remarkable among 

 the felspathic rocks ; whether, like granite, they are originally crys- 

 talline, or, like millstone-grit, composed of fragmented masses. The 

 felspathic portion of the hypersthene-rocks of Carrock Fell is so 

 w^asted that the crystals of hypersthene and magnetic iron are pro- 

 jected from the surface considerably. Some greenstone-dykes are 

 thus entirely decomposed to great depths from the surface, and 

 whole rocks of granite, secretly rotten, wait only for an earthquake 

 or water-spout to be entirely reduced to fragments. Those who have 

 seen the crumbled granite of Muncaster Fell in Cumberland, or 

 Castle Abhol in Arran, surrounded by heaps of its disintegrated in- 

 gredients, must have been struck by the importance of this pheno- 

 menon in reasonings concerning the origin of many stratified rocks." 



Where granitic or other felspathic rocks form moTintain-masses, 

 they have often been shattered in the elevation of the region ; and 



