MAGNETIC ZONES A BASIS OF ROCK CLASSIFICATION. 277 



dioxide (COJ, it certainly does not come from a temperature where that 

 gas could not exist. White mica is also characteristic, and it is decom- 

 posed at a full-red heat. Hornblende, which is quite characteristic 

 of the medium rocks, has been formed at 550° centigrade or a red heat ; 

 but at a not much higher temperature, in the j)resence of iron, steam is 

 dissociated, and hornblende might be burnt brown. Here, then, may be 

 the limit of the neutral magmas. 



Below this we shall have the basic zone, containing not steam, but 

 free hydrogen, and extending down to a white heat. This might require 

 a depth of 40,000 meters, say, twenty-five miles, which is also about the 

 limit of earthquake depths. Somewhere below this depth there would 

 come a limit to the expansive power of gas to raise a column of lava.* 

 and from below this level there could be no direct feediug, although of 

 course the gases themselves might work out. 



Basis of Rock Classification. — It is to be understood that these depths 

 and zones are put in the table for the purpose of clarifying our concep- 

 tions, and that the theory is not to be burdened with a rigid interpreta- 

 tion of them. For example, we liave entirely neglected pressure as in- 

 fluencing reactions, yet since Iddings has noted that the igneous rocks 

 show little trace of the effect of pressure, pure and simple, possibly we 

 may not be so far from the truth. Long ago it has been remarked that 

 the three factors in the cr3^stallization of a rock are the loss of pressure, 

 temperature and of included gases, the last two being by far the most 

 important. Let us emplo}^ sym])ols, as in the case of the three pinacoids 

 of a crystal, to which shall be assigned the following values : F = rate of 

 loss of pressure ; M (minera lisers) = rate of loss of absorbed gas ; 

 1"= rate of loss of heat. Then the coarseness of a texture will increase 

 with the slowness of the loss, the kind of texture with the relative rate 

 of loss. jTless than l/will be characteristic of the glassy textures and 

 volcanic rocks, rocks potentially amygdaloidal, and T greater than Jf of 

 the dike rocks in general, the pegmatites and veins. The plutonic rocks 

 will generally have T equal or, rather, concomitant with Jf. If we use 

 the sign = to indicate concomitant variation and > to indicate more 

 rapid loss, we may classify the rocks as follows : 



Abvssal, M = T = P. Crust of the earth, fundamental or batholitic 



gneiss (?), allotriomorphic texture (?). 



>/\ Hypidiomorphic and cement textures. 



<P. Miarolitic texture. 

 Intrusives, M <, T = P. Veins filled by ascent. 



<P. Panidiomorphic dikes. 



> P. Pegmatites. 



* But it should be remembered that the weight to be lifted is of a column of foaming lava full of 

 gas. 



