JONES ON THE WEATUEKINO OF liKANlTE 
'607 
weaker, action of these processes we see already producing effect on 
the exposed granite-faces of the old quarry at its foot (fig. 6) . 
Granite (like other rocks of igneous origin, and like many of 
aqueous formation also) is always cut through by joints or fissures 
nioi-e or less regularly, and these have probably originated in the 
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 granite 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 washed away in the form 
of white clay, the material so useful to porcelain-manufacturers, and 
prepared artificially to a lai'ge 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., 1865, p. 468) : — 
" The exterior of most uncrystalline rocks and buildings seem to 
be slowly eaten away by the moisture 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 
wasted that the crystals of hypersthene and magnetic iron are pro- 
jected from the surface considerably. Some gTeenstone-dykes are 
thus entirely decomposed to gi-eat 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 mountain-masses, 
they have often been shattered in the elevation of the region ; and 
