REPORT OF THE DIRECTOR I9II 167 



difficult or impossible to draw any line' between the original and the 

 injected material. The pegmatites themselves are derivatives of 

 the great intrusions of granite-gneiss referred to above as sur- 

 rounding and cutting through the Grenville, injecting it with every 

 variety of intrusion from batholyths down to sheets of almost 

 microscopic thinness. As shown in an early report,^ where the 

 intrusions become very thin they nearly always assume a pegmatic 

 habit. This apparently anomalous behavior doubtless finds its 

 explanation in the fact that only the more fluid portions strained off 

 from the magma are capable of penetrating the narrow openings 

 where, in virtue of their abundant gases and mineralizers, they 

 naturally crystalhze as pegmatites. It does not of course follow 

 that no thick masses of pegmatite are present, since these are by no 

 means rare, both types occurring with the ores. 



It is hardly necessary to recall at this point the fact that the 

 Grenville rocks are all completely metamorphosed by the combined 

 action of orographic disturbances and igneous intrusions, under 

 heavy cover. The tendency of the pyrite to occur as lenses, 

 conforming in general with the prevailing dip and strike of the 

 associated gneisses, in so far as it is the result of mechanical forces, 

 rather than replacement of lense-shaped masses, is doubtless to be 

 referred to disturbances subsequent to the period of intense meta- 

 morphism. 



Among the more intimate features of the ore deposits mention 

 should be made of the very general association of graphite with 

 the pyrite, the former mineral occurring at all the mines, often 

 in abundance though not evenly distributed through the ores. Thus 

 while pyrite ores commonly carry graphite, this is not invariably the 

 case. Conversely graphite, which is widespread in Grenville rocks, 

 often occurs without pyrite. 



Another feature of the ores is the nearly universal occurrence of 

 a chloritic alteration product with the pyrite. This material is 

 evidently derived from mica in some cases, but in others seems 

 equally clearly to grow at the expense of other minerals, particularly 

 quartz and feldspar. Such an alteration is by no means general in 

 the Grenville rocks, but on the contrary appears to^ be the result of 

 unusually potent chemical agents associated with the formation of 

 pyrite. 



1 Smyth, C. H., Jr. Report on the Crystalline Rocks of St Lawrence 

 County. N.Y. State Museum, 49th Ann. Rept. for 1895, H. p. 491- 



