65 NEW YORK STATE MUSEUM 



As compared with the amphiboHte, the granite must be regarded 

 in all these occurrences as possessing a certain degree of iluidity, 

 while the former, in spite of the incipient melting often produced 

 by the heat of the magma on thin exposed edges, was substantially 

 in a solid and relatively brittle state. The difference in the physical 

 condition of these two rock types is in fact so great as entirely to 

 exclude the possibility of the amphibolite being broken dikes. This 

 hypothesis, as Professor Smyth showed, necessarily implies that 

 both the granite and the amphibolite were solid, and under dynamic 

 stress they would have a coefficient of viscosity of the same order 

 of magnitude. It is therefore impossible to admit that in the zone 

 of flowage the amphibolite could retain its brittle character if the 

 granite yielded to the pressure to the extent of developing the 

 fluidity indicated in the outcrops described. The former, on the 

 other hand, would be affected by the pressure in approximately the" 

 same degree as the granite, and would be pulled and drawn out into 

 a long lenticular form, as is the case with the amphibolite sheets 

 intruded into the garnet gneiss of the sigmoid area. In general, 

 however, the effect of the magmatic heat has been gradually to 

 soften these inclusions, and in some cases, especially the smaller 

 broken pieces, they have assumed a fluidity comparable with that of 

 the granite. Then, on being drawn out by magmatic movements 

 into long thin streaks resembling " schlieren," the xenolithic ma- 

 terial has lost all those features by which its original solid character 

 can be recognized. Hence has arisen the universal difficulty of 

 establishing the true nature of the amphibolite masses; but it is 

 believed that the indications afforded by those rare occurrences 

 where the xenoliths are in the act of breaking apart, afford a basis 

 for supposing that much if not all of such material was derived from 

 the walls of the magmatic chamber, and is not broken and squeezed 

 dike material of younger age than the granite, as was maintained 

 in earlier hypotheses. 



With this much gained as to the source of the material, there 

 still remains a very troublesome question as to the original nature of 

 the inclusions, that is, the lithologic prototype, which on meta- 

 morphism gave rise to the abundant amphibolite now found as 

 xenoliths. In the present quadrangle the general relationships at 

 certain points, as along the granite border east of Eddy, strongly 

 suggest that they are fragments of hornblendic contact-metamorphic 

 zones, caught up during the emplacement of the granite. Unfor- 

 tunately most occurrences, especially those away from the im- 

 mediate border of the granite, show ambiguous .rebjtioii^hips, and 



