192 MINERALOGY AND LITHOLOGY. 
the microscopic geologists of later times, have developed the theory of 
igneo-aqueous fusion to account for the formation of granite, a theory 
about which so many observations have been accumulated that its truth 
is nearly demonstrated. 
In granites, the feldspar is almost always troubled by the inclusion of 
impurities, by rifts and decomposition products, which in thin sections 
throw it into striking contrast with the clear quartz, which, though free 
from the cleavage and enclosures of the feldspar, always contains the 
cavities which are filled with water, and a bubble, and often crystals of 
salt.* The quartz, as the last mineral to crystallize, of necessity took in 
the fluid residue, and, being difficult to decompose, has held it. Sorby 
has been led by his observations on these cavities in granites to believe 
that the temperature at which the quartz finally solidified is that which 
marks the critical point when compressed steam passes to water, which 
is at about the melting point of zinc (412 C). This is based on the 
observation that the few cavities that exist in the feldspar of certain 
granites contain but little water and no separated salts. This indicates 
that the feldspar crystallized at a temperature above the critical point 
of water, which in a gaseous state cannot dissolve salts, and hence the 
cavities are nearly empty and destitute of crystals. The abundant pres: 
ence of salts suspended in the water contained in cavities in the guartz 
shows that the quartz, on the contrary, was still plastic at the tempera- 
ture below the critical point of water, which as a fluid, when highly 
heated, is a most powerful solvent, and hence would dissolve soluble 
constituents from the other minerals, and on cooling it would be a super- 
saturated solution. This temperature agrees very closely with that de- 
duced from his calculations made upon the measurements of the relative 
size of bubble and cavity. It is interesting, however, in this connection, 
to note that the temperature of 412 C. is the point at which substances 
possess a dull red heat; and Scheerert deduced this same result from his 
studies on the pyrognomic minerals held in granite. He considered that 
the existence of certain minerals, such as gadolinite, allanite, &c., which 
at a temperature not above a brown red suddenly disengage heat, glow 
* See p. 183. 
} Mineralogical Magazine, Nov., 1876. 
{ Bul. de la Société Géologigue de France, second series, 1846, vol. 4, p. 487. 
