82 ANNALS NEW YORK ACADEMY OF SCIENCES 



four to six per cent. The specimens of schist analyzed had undergone 

 little or no contact action; but it will be recalled that at Crugers, for 

 example, the schist shows a steady increase in alumina as the igneous rock 

 is approached, until on the contact the per cent, is 29.50, as shown by 

 "Williams's analyses. The same, moreover, holds with regard to iron; the 

 unaffected schist runs 7 or 8 per cent., but at the contact shows 25 per 

 cent. Even at a point 700 yards from the contact, the schist runs 24.32 

 per cent, in A1 2 3 and 11.12 per cent. FeO and Fe 2 3 . The reason 81 for 

 this increase is a question beyond the province of this paper; the fact 

 remains that the mica schist in contact with the igneous rock is abund- 

 antly able to contribute alumina and iron and that even the unaltered 

 schist is much higher in alumina than the most common country rock of 

 the emery. The silica of the schist presumably goes to form the quartz 

 streaks so generally found with the emery. That the emery may be formed 

 by the absorption of schist is also indicated by its occurrences in the cliff 

 of diorite at Crugers. As described above, every gradation, from the 

 unaltered mica schist to one composed chiefly of quartz, biotite, magnetite, 

 sillimanite, garnet, etc., at the contact and so to the inclusions in the 

 igneous rock itself, composed of magnetite, spinel and corundum, may be 

 traced. These thin lenses of magnetite and spinel are so completely 

 changed that they were once mined for iron. Moreover, pleonaste was 

 found in the garnet rock from the contact at Salt Hill ; and it was found 

 in one case in an igneous dike cutting limestone. 



The sillimanite, etc., with which the emery is associated, may, accord- 

 ing to Morozewicz, be found when the ore is pyrogenic, although 1ST. H. 

 and A. N. Winchell state that sillimanite "in igneous rocks occurs only 

 as a result of absorption of foreign material." 82 It is at least certain that 

 it occurs abundantly on the borders of the district, where it can only be 

 due to contact action. Garnet, which is also found as a reaction mineral, 

 is a similar case. The great abundance of biotite in some of the mines is 

 paralleled by its abundance at the Crugers contact, and the green mica 

 which is said to occur sometimes in the emery is also found by Williams 

 in certain inclusions and identified by him as margarite. Quartz is com- 

 mon on the contact and also constitutes the bulk of some of the inclusions 

 at Crugers ; its association with the emery is thus paralleled, whereas it is 

 apparently not known in association with true pyrogenic corundum. 



If this theory be correct, anything like a sharp contact of the emery or 

 its associates with the mica schist would not be expected, nor, in fact, 



81 There seems to be no warrant for thinking that the igneous rock coulrl have con- 

 tributed this iron and alumina, as is often the case in garnet zones in limestone. 



82 Optical Mineralogy. N. Y., p. 363. 1000. 



