Veins in Sienite. 



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say sienite, was in an incipient state of refrigeration, matter of a sim- 

 ilar kind, still more intensely heated, might have been' injected into it 

 so as to form veins. And the very near approach which some of the 

 veins in sienite and granite exhibit to veins of segregation, has led me 

 to the suspicion that they might have been produced in some 

 such way, rather than by the filling up of open fissures subsequent to 

 the consolidation of the mass of the rock. However, very many of 

 the veins in our sienite were obviously produced in the mode last 

 mentioned : for we find that lateral slides, sometimes of considerable 

 extent, have taken place along the course of the vein ; and this could 

 not probably have been done till after a considerable degree of indu- 

 ration. 



No substance is so common in the veins of our sienite as epidote. 

 Yet the veins of this description are far less conspicuous than oth- 

 ers, because they are so thin. Often they are easily mistaken for 

 mere seams. The epidote is commonly very compact and resembles 

 greenish compact feldspar : it has not, however, the toughness of that 

 mineral, and it melts rather easily before the blowpipe into a black 

 enamel. Sometimes the epidote of these veins is semicrystalline and 

 well characterised. I think that the lateral slides in the sienite men- 

 tioned above, occur more frequently in connection with these epidote 

 veins than with any other. 



The other substances composing veins in our sienite, are granite, 

 feldspar, and quartz. The granite is most common : though generally 

 feldspar is greatly in excess, and frequently no mica is present. Hence 

 the vein by a slight change becomes entirely feldspar, ordinarily of a 

 reddish hue. In one instance only, which has been already mentioned 

 under porphyry, a vein two or three feet wide consisted entirely of 

 compact feldspar. The quartz veins are frequently hollow and 

 abound in delicate crystals of the same mineral. 



By far the most instructive exhibition of the veins that have been 

 described, occurs in Whately, a mile or two a little west of south from 

 the congregational meeting house. I subjoin a few sketches of those 

 that struck me as particularly interesting. 



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