GEN ERA L REMARKS ON GNEISS. 221 



chiefly in the larger size and more decided crystal- 

 lization of their constituent parts. It is in these that the 

 large concretions of mica and of felspar are almost always 

 found. I have rarely observed that in the granite veins 

 of gneiss there is any difference of texture between the 

 middle and the exterior parts, although in those that tra- 

 verse other rocks and can be traced to granite masses, this 

 feature is common. The same rule holds respecting 

 the larger and smaller veins, which in gneiss possess indif- 

 ferently a similar texture, while in other rocks the texture 

 is minute, generally, in proportion to the vein. Many 

 of the veins in gneiss consist of felspar and quartz with- 

 out mica or hornblende ; in all, these minerals are pre- 

 dominant. 



Graphic granite is much more frequent in the veins 

 that traverse gneiss than in the others, although not 

 absolutely limited to these. The felspar is generally the 

 predominant substance in this class of veins and often 

 presents a common polarity throughout the whole mass, 

 as already noticed in Coll. The mutual disposition of 

 the felspar and quartz is various. Occasionally it is 

 partially laminar, as in Rona ; more commonly the quartz 

 is in prismatic forms, triangular or hexagonal, or occa- 

 sionally, even hollow and filled with felspar. In a very 

 few instances the summits of the quartz crystals are per- 

 fect, and protrude into a vacant space. From considering 

 the relative forms of the quartz and felspar it will some- 

 times appear that the one and sometimes that the other 

 has first crystallized, and thus determined the shape of 

 its associate. 



Gneiss, considered as a rock, alternates with many 

 substances generally inferior in quantity, to which the 

 arbitrary term subordinate has been applied. Sometimes 

 however it is inferior in quantity to those very rocks, 

 and might therefore with equal justice be considered the 

 subordinate substance. It is difficult to draw a sufficient 

 line of distinction between those alternations which may 



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