132 PROCEEDINGS OF THE AMHERST MEETING 



Many narrow tongues of granite extend partly into the lenses parallel to 

 the foliation : many lenses are cut across at low angles by narrow bands of 

 granite, and some small shreds or tiny lenses have been separated 1 or 2 milli- 

 meters from their parent lenses. 



The granite maintains its uniformity of grain and color up to the very con- 

 tacts against the lenses. To the naked eye. these contacts look fairly sharp. 

 Thin sections across the contacts, however, show that, while the composition 

 of the granite is maintained, there are usually some scattering needles of 

 sillimanite in the granite from 1 to 6 millimeters from the contacts. 



The inclusions Quite certainly represent fragments of the sedimentary Gren- 

 ville quartzite or quartz-schist formation. This conclusion is based not only 

 on the composition, structure, general appearance of the lens material, and the 

 presence of quartzite in various parts of the quadrangle, but also on the fact 

 that a large exposure of quartz-sillimanite gneiss or schist lies along the west- 

 ern border of the southern area. This nearly white rock forms a high, wide 

 ledge fully one-eighth of a mile long. It is in every essential way like the 

 lens material, except that it carries somewhat less sillimanite and is not so 

 highly foliated. The dip and strike of its foliation are parallel to those of 

 the adjacent lens-bearing granite. Some small pegmatite and silexite dikes 

 cut the ledge parallel to its foliation. A thin section of a specimen from this 

 ledge contains the following minerals by volume percentages : quartz, 77 ; 

 sillimanite. 20; magnetite, 2: hematite, 1; and a little zircon. Most of the 

 sillimanite is in the form of badly decomposed long prisms, but a good deal 

 of it fills irregular cracks in the quartz. 



The history of the above-described phenomena is believed to be essentially 

 as follows : The granite magma entered two long belts of Grenville well bedded 

 quartzite. forced its way between, and wedged apart the layers, and broke 

 them up into myriads of lenses, all of which became arranged more or less 

 perfectly parallel to the magmatic currents. It is a remarkable fact that the 

 granite magma should have so thoroughly and systematically broken the old 

 quartzite formation up into such a vast number of small lenslike fragments, 

 practically none of which are more than 1% feet long. In many cases narrow 

 tongues of magma were forced across lenses ; in many cases narrow tongues 

 of magma were forced lengthwise partly into the lenses, and in some cases 

 tiny lenses or shreds were pulled away from the larger lenses 1 or 2 milli- 

 meters into the magma. The lenses were subjected to considerable pressure 

 in the cooling granite magma, and this probably accounts for their highly 

 toliated structure. 



Because the quartzite was so thoroughly and rather uniformly separated 

 into small fragments, and because the lenses constitute relatively little of the 

 total volume of rock of each of the two areas, the granite magma, during its 

 consolidation, was little or not affected by the lenses, and it therefore crystal- 

 lized with very uniform grain, color, and composition, even up to the very 

 contacts against the lenses. The magma appears to have been totally unable 

 to really assimilate any of the quartzite. 



Since sillimanite needles commonly lie in the granite several millimeters 

 out from the lens contacts, where they show exactly the same relation to the 

 granite that they do to the lens material — that is, they usually lie between 

 the mineral grains, but in some cases they are inclusions in them — it is prob- 



