90 



It is evident that microscopic examination throws light not 

 only upon the origin of such quartz-bearing rocks as granite, but 

 also upon that of quartz as an accidental and accessory mineral 

 among rocks, and upon the mode of formation of quartz veins. 

 We are in fact led to see that heated water containing silica in 

 solution has played a most important part in geological history, 

 that such water has sometimes been diffused through a rocky 

 magma at a dull red heat under enormous pressure, and finally 

 become entrapped in millions of minute cavities in the solidified 

 rock ; sometimes it has worked its way up along cracks and fissures 

 and deposited quartz in those fissures, forming veins ; sometimes 

 circulating throughout the mass of a rock it has deposited the 

 quartz in all irregular cavities or vesicles at a less degree of 

 temperature ; and finally, we recognize the same heated water 

 fully charged with silica rising to the surface in the form of the 

 Geysers of Iceland, and playing a large part in all volcanic out- 

 bursts. Surely there can be few thoughts more surprising than 

 this, that every piece of granite we pick up contains in its quartz 

 particles thousands of minute liquid-cavities, and moreover that 

 every such liquid-cavity includes a tiny vacuous bubble in constant 

 tremor or active motion, such motion, it would seem, having been 

 kept up for the countless ages since the granite was first solidified 

 deep down in the bowels of the earth. Truly we learn great things 

 from study of the most minute. 



