50 THE GRANITES OF MAINE. 
the weathered rock. The subject of weathering of granite is fully 
treated in the writings of Merrill, Keyes, and Watson." 
The changes in granite after it has entered into buildings or other 
constructions arc less marked than those in the natural rock, because 
the blocks arc not then traversed by anything analogous to sheet and 
joint structure, and also because the years of historic time are few 
compared to those of geologic time. Much has been written on the 
decay of granite in monuments and buildings.'' Such decay is mainly 
attributable to microscopic fissures produced by the unequal and 
repeated expansion and contraction of the different minerals of the 
granite under changes of solar temperature. In countries where the 
winter temperature is very low the action of frost within such fis- 
sures powerfully assists the process of disintegration. Thus the obe- 
lisk now in New York suffered more from three years' exposure to 
our climate than it had during over three thousand four hundred 
years in Egypt, although the fissures along which frost operated were 
started long before it reached this country. A minor factor in decay 
is the chemical action of water along fissures. It is supposed that 
these causes of decay operate more effectively in coarse granites than 
in fine ones. Merrill points out that a -awn or properly prepared 
polished surface resists weathering more effectively than a cut or 
hammered one. as the latter is full of minute fracture.-, parallel to the 
surface, produced by impact, which facilitate scaling. 
B~LACK GRANITES. 
BLACK GRANITES IN GENERAL. 
( LASSIFICATION. 
The term u black granites," although sufficient for general com- 
mercial purposes, include- a variety of rocks of different character, 
origin, and appearance — gabbros, diorites, diabase, etc. They have, 
however, three mineralogical features in common — they contain com- 
paratively little or no quartz, their feldspar belongs entirely 01 
almost entirely to the series which contains both soda and lime. an< | 
" Merrill, Goo. P.. Disintegration of the granitic rocks of the Districl <>f Columbia : Bull 
Geol. Soc. America, vol. 6, p. 321, 1895; also A treatise on rocks, rock-weathering, ;un 
soils. Now York. 1897, pp. 206 214. 236, 244, 245, 2:>7. Keyes, Charles R., The origli 
and relations of central Maryland granites : Fifteenth Ann. Rept. T r . S. Geol. Survey. 189G 
p. 725, and pis. 42-45. See also Proc. Iowa Acad. Sri., vol. 1. pt. 3, pp. 22-24, an 
vol. 2, pp. 27-31, Tls. II-IV, 1895. Watson, Thomas L., A preliminary report on a pal 
of the granites and gneisses of Georgia, 1902, pp. 299, 300, 308, 329, 331, ::::::. 
''.Tnlien, Alexis A.. The durability of building stones in New York City: Tenth Censil 
vol. 10, 1884; Granite, pp. 370-371. Merrill. Coo. P., Physical, chemical, and econom 
properties of building stones: Maryland Geol. Survey, vol. 2, 1898; Granite, pp. 92 '•» 
Also Merrill's Stones for Building and Decoration, 3d ed., 1903; Weathering of granil 
pp. 434. 435. 
'See Julien, Alexis A.. A study of the Now York obdisk as ;i decayed bowlder: Anna 
New York Acad. Sci., vol. 8, J Sit:;, pit. 93-166. 
