192 MINERALOGY AND LITHOLOGY. 



the microscopic geologists of later times, have developed the theory of 

 igneo-aqiieous 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 f 

 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 quartz 

 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 ScheererJ 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. 



f Mineralogical Magazitie , Nov., 1876. 



X Bui. de la Socictc Ceologigue de France, second series, 1846, vol. 4, p. 487. 



