618 WILLIAM J. MILLER 



biotite and hornblende. Mica formed first in the liquid mass. 

 At this time the orogenic pressure acted upon the peripheral zone 

 of the magma by orienting this mineral normal to the pressure. In 

 the heart of the viscous mass this faculty of orientation was 

 replaced by an interior tension not directed in any particular way." 1 

 Orogenic pressure did not exist in the Adirondack region, but if for 

 it we substitute the pressures within the stock magmas themselves, 

 this idea of Weinschenk affords a plausible explanation of the foliated 

 borders. Considerable pressures must have obtained within the 

 stock magmas which were intruded under very deep seated con- 

 ditions. Such pressure against the country rock, combined with 

 the usual development of differential flowage in the magmatic 

 borders, as already explained in this paper, would readily account 

 for the peripheral foliated zones which were produced, no doubt, 

 during a late stage of magma consolidation. But the conditions 

 for magmatic pressure and flowage must often have varied a great 

 deal, so that it is to be expected that, in some cases, even coarse- 

 grained gabbro would exhibit primary foliation, while, in other 

 cases, amphibolite would make up the whole mass, or, in still other 

 cases, finer-grained, very gneissoid, and granulated belts or bands 

 would occur in the midst of coarser, less foliated types. It should 

 be noted in this connection that unmistakable flow structures do 

 often occur around inclusions in the gabbro. 



Applying these ideas, the puzzling features of various gabbro 

 stocks find a ready interpretation. A good example is the stock 

 near Blackbridge in the Lake Pleasant quadrangle. 2 For most 

 part this is a very basic, gabbroic-looking rock, sometimes pretty 

 massive and very coarse-grained, and at other times not so coarse, 

 but streaked or almost banded, owing to layers of amphibolite. 

 All phases of the rock are much granulated and distinctly gneissoid, 

 the coarser-grained portions being least so. A diabasic texture 

 frequently occurs. Differential flowage and other movements 

 under pressure in the congealing magma best explain these phenom- 

 ena. The more foliated, finer-grained belts in the midst of the 



1 E. Weinschenk, Congres geol. inter., Compte rendu, Session VIII, i (1900), 340. 

 Freely translated from the French. 



2 W. J. Miller, New York State Mus. Bull., No. 182, 1916, p. 29-30. 



