PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 31 



more or less shifted after they were developed, by movements in 

 the granite magma, which still possessed a considerable degree of 

 fluidity. Such breaking up and shifting of pegmatite and silexite 

 masses appear to have been most notably true of the silexite. 

 Many of the later pegmatite dikes (below described) exhibit no 

 such phenomena. We are led to conclude, therefore, not only 

 that many masses of practically pure silica (silexite) separated 

 from the granite magma well before many of the ordinary peg- 

 matites, but also that the separation of the silica took place well 

 before the complete solidification of the granite magma. Accord- 

 ingly those masses of silexite which developed early (probably as 

 segregation masses) and were broken up and more or less shifted 

 in the magma are to be regarded as inclusions in the granite. 



The following sketches (Figs, i, 2, 3) of portions of the ledges 

 at the summit of Duncan Mountain will suffice to make clear the 

 foregoing conclusions. Figure i shows seven fragments of silexite. 

 Evidently they represent portions of one or more masses which 

 were broken up and strung out in the magmatic current. The 

 wrapping of the magmatic flow-structure about the fragments is 

 very evident. Contacts of the silexite against the granite are 

 sharp. That these fragments are true inclusions of silexite which 

 must have developed (probably by segregation) well before the 

 final consolidation of the granite magma is strongly supported 

 by the undoubted inclusions of both Grenville strata and an old 

 gabbro which, in the Lyon Mountain district, have been observed 

 to bear exactly similar relations to the granite (see Fig. 5). Unlike 

 the Grenville and gabbro inclusions, however, the fragments of 

 silexite are not distinctly older than the granite. They are early 

 facies of the pegmatitic masses which developed during the intru- 

 sion of the granite and probably after the magma had come nearly 

 to rest. Thus other nearby ledges of granite at the top of Duncan 

 Mountain show distinct inclusions of silica which contain varying 

 amounts of feldspar, though seldom more than 15 or 20 per cent. 

 Usually with increased feldspar content the contacts against the 

 granite are less sharp. Also in many cases silica, usually with a 

 small content of feldspar, still exists in rather distinct dikelike 

 form with only fairly sharp contacts against the granite. Again, 



