54 THE GRANITES OF MAINE. 
flowed along the sheet partings and permeated the rock above and 
below them. This staining near the surface is intimately associated 
with the " shake " structure, which may be the result of frost. 
Whether the postglacial submergence of the Maine coast had any- 
thing to do with the discoloration is not clear. 
Another kind of discoloration, which is even more serious in its 
consequences, appears on fresh faces of granite, either in the quarry 
or after its removal. This consists of sporadic rusty stains from 
half an inch to 1 inch in diameter, arising from the oxidation of 
minute particles of some undetermined ferruginous mineral. In the 
Maine quarries these limonitic spots arc very exceptional. 
Daly a describes a bluish-gray syenite (feldspar, quartz, hornblende, 
augite, biotite) that after twenty-four hours' exposure assumes a 
greenish tinge, which eventually becomes more or less brownish. He 
has demonstrated by experiment with oxygen that this change is due 
to the oxidation of minute blackish granules of ferrous oxide within 
the feldspar, giving a yellow which, in combination with the original 
bluish tint of the feldspar, produces a green. The large columns of 
the library of Columbia University, in New York, are made of this 
rock. Such changes, however, are uncommon in granitic rocks. The 
only similar one observed in the Maine granites was in the quartz 
diorite of Alfred, in York County. (See p. L75.) 
Another kind of discoloration occurs on either side of diabase 
or basalt dikes, consisting mainly of various alterations of the feld- 
spars, and their consequent change in shade or color. (See p. 91.) 
Discoloration is thus of four kinds: Thai due to the infiltration of 
ferruginous water, that due to the oxidation of sporadic ferruginous 
minerals, that arising from the oxidation of ferrous oxide within the 
feldspars, and that i\\w directly or indirectly to dikes and veins. 
To these should be added a possible fifth — that due to the oxidation 
of the generally disseminated ferruginous minerals (biotitej horn- 
blende, magnetite) by non ferruginous water. 
DECOMPOSITION. 
Notwithstanding the strength and durability of granite, it is liable, 
under certain conditions and in the course of long time, to decompose 
into. a clayey sand. This is the result of its physical, mineralogical 
and chemical constitution and properties. One of the most striking 
illustrations of this is the occurrence in some of the Maine quarries 
of "beds" of sand or decomposed granite within the fresh granite, 
either between the sheets away from headings or within the headings 
" Daly. Reginald A.. The geology of Ascutney Mountain, Vermont: Bull. U. S. Geol. Sur- 
vey No. 209, 1903, j»i>. 51-53. 
