DECOMPOSITION. 55 
and along or across the sheets. Thus at the Palmer quarry, in Vinal- 
baven, 20 feet below the surface in the face of the quarry there is a 
bed of granite sand 18 inches thick between two sheets, which at 
that point dip about 10° into the hill. On the southeast side of the 
Longfellow quarry, near Hallowell, some of the sheets within a wide 
heading include granite sand beds 10 inches thick. At the Shattuck 
Mountain quarry, near Redbeach (see p. 165) a G-foot heading in- 
cludes a vertical layer of granite sand 8 inches thick. Specimen- 
taken from these various sand beds show that the disintegration be- 
gins with miscroscopic fractures; in some cases the enlarged rift 
cracks, producing the " shake " structure described on page 40, and is 
followed by more or less kaolinization of the feldspars. This process 
consists in the loss of alkali and the taking up of water, resulting in 
the passing of the feldspar into a whit-e clay (kaolin). 
The joint and sheet structure affords ingress to surface water, con- 
taining its usual percentage of carbonic acid, and the " rift " or 
" shake " structure facilitates the kaolinization of the feldspar on 
either side of the sheet parting by this water. As the feldspars pass 
into clay the rock crumbles into sand consisting of quartz, mica, and 
kaolin, and of feldspar in various stages of kaolinization. In some 
places within the range and depth of frost a large part of this work 
may have been done by frost alone. The sand would there be mainly 
the product of the " shake " structure. 
In regions which have not been swept by a continental glacier 
any granite mass would be covered with the products of the decompo- 
sition of its own surface. In the Tropics the abundant rainfall 
and the organic acids from a luxuriant vegetation materially hasten 
the decomposition, and granitic rocks in such regions are for these 
reasons often covered with many feet of sand and soil." Along the 
Maine coast the surface of granite ledges bear in protected places an 
inch or so of granite sand, which represents surface disintegration 
since the postglacial submergence. 
The incipient stage of weathering may be observed in any long- 
exposed granite ledge in the milky whiteness of the feldspars. This 
change usually attacks the soda-lime feldspars first. The black mica, 
owing to its content of iron oxide, is also liable to early decomposi- 
tion. The process of weathering, as it affects the rock as a whole, 
involves the following chemical changes: A loss of lime, magnesia, 
potash, and soda; a gain of water, and a relative gain of silica, 
alumina, and iron oxide — that is, relative to the reduced weight of 
° Branner, op. cit., p. 31. 
