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GRANITE. 



the principal arguments respecting the posteriority of 

 granite to the strata with which it is associated. 

 These vary infinitely in their dimensions, extent of 

 course, entanglement, and ramifications. At times 

 they are rather protuberances from the general mass, 

 than veins ; while at others, they extend to great dis- 

 tances ; insinuating themselves widely into the sur- 

 rounding strata, above all, in gneiss, in which rock 

 also they especially abound. Thus also their thick- 

 ness varies, from many yards, even to the minuteness 

 of a thread, being simple or ramifying; and often also 

 presenting the most intricate reticulations. In com- 

 position 5 the larger veins at least, sometimes resemble 

 the parent mass, while, in the smaller, the structure 

 often becomes minute, as if proportioned to the mag- 

 nitude of the vein. But, more commonly, the mate- 

 rials are crystallized on a much larger scale, producing 

 the well-known specimens of felspar and mica: as, in 

 these veins also, the accidental minerals enumerated 

 in the Classification are chiefly found. Yet the size 

 of the ingredients does not bear a proportion to that 

 of the vein ; the larger crystallizations as often occur- 

 ring in the small as the large ones. 



It is also not unusual for the mica and the horn- 

 blende to disappear in the progress of the smaller 

 veins, while quartz and felspar are always the predo- 

 minant ingredients : as, further, even the quartz often 

 disappears, so that the ulrnitate ramifications consist 

 solely of\ felspar. And though hornblende is as com- 

 mon an ingredient as mica in gneiss, it rarely occurs 

 in the veins that traverse an example of this variety, 

 though mica is present in those which are found in 

 micaceous gneiss. Occasionally, and even in a large 

 vein, all the crystals of felspar preserve a common 

 polarity, however interrupted by other minerals ; so 



