3^4 Scientific Geology. 



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Veins in Quartz Rock. 



In a few instances, as at the quarry of quartz rock in Washington, 

 several times spoken of, veins of granite may be seen. But gener- 

 ally the veins in this rock are quartz ; the vein being usually white 

 and opaque, and the rock a mixture of gray granite and mica, the 

 latter mineral existing, however, in very small proportion. In some 

 instances the rock appears to be what the Wernerian writers denomi- 

 nated primitive siliceous slate. The following is a sketch of a 

 bowlder of about eight inches diameter, found in Amherst. The 

 principal part of it is gray quartz traversed by numerous veins of 

 white quartz. 



Quartz Veins in Quartz Rock. 



Theoretical Considerations. 



The regularity of the stratification in quartz rock, and the fact that 

 silica is soluble in water, have disposed geologists, in all cases where 

 it is possible, to impute to this rock an aqueous origin. But like all 

 the older rocks it appears subsequently to have been subjected to heat 

 of a greater or less degree of intensity, whereby it has been rendered 

 compact. And no doubt in this way some siliceous sandstones have 

 been converted into solid quartz : as in the Isle of Sky in Scotland, 

 where trap comes into contact with the sandstone. 



A complete theory of the formation of that variety of brecciated 

 quartz, which in Dalton is cemented by hydrate of iron, it is not easy 

 to form. The chief difficulty seems to be, to imagine how the quartz 

 was broken into such numerous angular fragments : for after these 



