SO-CALLED PORPHVYRITIC GNEISS OF NEW HAMPSHIRE 785 
than one phase of a magma." It is conceivable that it might be 
locally brought about by the pulling out of basic segregations 
in a plutonic rock by mechanical force operating during or after 
complete consolidation. 
On the road along the north shore of Wickwas pond a strik- 
ing phase of the porphyritic granite was found which had been 
discovered before but never in such perfect development. It 
forms a strongly schistose mass a few inches thick which lies 
parallel to the foliation of the normal rock. Composed largely 
of brown biotite with here and there a large feldspar phenocryst, 
it is very different in appearance. With the biotite large apatites 
and considerable masses of titanite and magnetite make up the 
groundmass. This band had much the appearance of a shear- 
zone, like that described in the crystallines of the Malvern Hills.” 
But the microscope discloses no strong evidences of crushing in 
the feldspars which are clearly original or primary in their nature. 
Now there are plenty of cases on record where biotite segrega- 
tions in granitic rocks grow to large size. Those at Graniteville, 
Missouri, vary from a few inches to five feet or more in diameter. 
It may be that this and similar local bands in the rock in question 
are due to the tailing out of such segregations before the final 
solidification of the whole rock had set in. The resulting bands 
would thus be parallel to neighboring structure-planes and take 
their place as primary elements in a fluxional mass. 
But the most favorable chance for the exhibition of a parallel 
structure over large areas would be given in cases where there 
is more than one generation of minerals; 7. ¢@., in porphyritic 
rocks. Such, indeed, has been the character of most of the plu- 
tonic rocks where extensive flow-structure has been described. 
The large size of the phenocrysts of the porphyritic granite is one 
of the chief conditions leading to this peculiar and widespread 
foliation. Possessed of large growth before the final magmatic 
crystallization set in, each feldspar phenocryst was, as it were, 
*HARKER and MARR, Q. J. Geol. Soc., 1891, p. 283; A. GEIKIE and TEALL, 
ibid., 1894, p. 656. 
© Geol soc. 1580s pai4i7 7 
3Geol. Surv. Missouri, Ann. Rep., VIII, 1894, p. 154. 
