20 THE GRANITES OF MAINE. 
hydrogen, and nitrogen in the following combinations and propor- 
tions :\X),, 23.60; CO, 6.45; CH 4 , 3.02; X,. 5.13; H 2 , 61.68.° These 
gases are probably present in all granites. 
rr.vn in:. 
Definition. — By the texture of a rock is to be understood those char- 
acteristics which are apparent on it- surface, or, more exactly, the 
forms and mutual relations of its minerals as seen without and with 
a microscope. 
Character and grade. — The most important feature of granite is 
the character of its grain. Sonic granites are even grained; others 
contain more or Less thinly disseminated and complete crystals of 
feldspar in a ma>s of liner, even-grained materia] -that i^. they 
show what is called porphyritic texture. The next most important 
feature— perhaps more Important —is the relative coarseness or fine- 
ness of grain in an even-grained granite. Three grade- of texture 
of this sort may be distinguished: ( 1) Coarse, in which the feldspars 
generally measure over 1 cm., or two-fifths inch; (2) medium, in 
which they measure under 1 cm. (two-fifths inch) and over <>.:> cm. 
(one-fifth inch) : (3) fine, in which they measure under 0.5 em. (one- 
fifth inch). In souk 1 coarse-grained granite- the feldspars measure 
one or several inches, and in some fine-grained ones all the particles 
range from 0.25 mm. to 1 mm. (One twenty-fifth inch) in diameter. 
and some average as low as 0.50 mm., or one-fi ft Let h inch. Ext remely 
fine ones average 0.175 mm., or about seven one-thousandths inch. 
Forms of minerals. — Even without the aid of the microscope it will 
be noticed that, except in granites of porphyritic texture, the minerals 
rarely attain their complete crystalline form. They have interfered 
with one another's growth. It will also be noticed that some of the 
crystals of feldspar in some granites are surrounded by a border of a 
different feldspar. Thus a red feldspar may be bordered by a white 
or greenish one, or vice versa. Kit her of these may be the potash 
feldspar and the other a soda-lime feldspar. It will also be noticed 
that many of the feldspars are not simple incomplete or complete 
crystals, but "twins," having the cleavage planes in one half at a 
different angle from those in the other half, so that when held in the 
sunlight only one half will reflect the light in one position. 
Arrangement of minerals. — A polished surface of any medium or 
coarse-grained granite shows that the quartz (ill- up the spaces be- 
tween the feldspars — that is. was formed after them— also that 
both feldspars and quartz inclose particles of mica, etc.. which must 
therefore have crystallized before them. Under the microscope the 
arrangement of the minerals is found to be such that they must 
"Tilden, W. H., Proc. Roy. Soc. Loudon, vol. 60, No. 366, Feb. 20, 1897, pp. 454, 455. 
