26 ANNALS NEW YORK ACADEMY OF SCIENCES 



is green hornblende, to one which carried biotite alone, and in the field, 

 moreover, the two types are very intimately related. The whole of the 

 large area of diorite is of this type, except in two places along the contact 

 of the diorites with the pyroxenites, where the hornblende is brown, the 

 rock carrying biotite as usual. All of the smaller areas of diorite carry 

 green hornblende excepting two, — the patch just southeast of Pleasant- 

 side, which appears to grade into the associated hornblende norite, and 

 the northern part of the area southeast of Salt Hill, which appears to be 

 closely associated with the neighboring biotite augite norite, both carry 

 the brownish variety. This latter patch, however, shows green horn- 

 blende along its southern margin. Williams's conception of the affinities 

 of these two main types, derived from the study of a small portion of the 

 district, appears to hold therefore for the whole series. 



On Montrose Point, the brown hornblende is most plentiful, both in 

 the hornblende pyroxenites shown on the map and in associated diorites 

 which are not mapped. The extraordinary complexity of the rocks on 

 Montrose Point makes the geology impossible of adequate representation, 

 and it was deemed advisable, therefore, to map merely the most abundant 

 rock, the pyroxenites. To quote from Williams: 25 ''These rocks (horn- 

 blendites) have a glistering black color and are most intimately asso- 

 ciated with the norites, hyperites, diorites and pyroxenites which also 

 occur there. No more complicated interpenetration of eruptive rock- 

 types could possibly be imagined than is displayed at this locality — every 

 rock includes and forms dikes in every other ; and at the same time, every 

 type passes by gradual changes in its mineralogical composition into 

 every other one." It is here, then, that the brown hornblende is best 

 developed; and in this wonderfully complicated net-work of rocks, it 

 grades on the one hand into a biotite augite norite and on the other into 

 hornblendite and hornblende pyroxenite. In the latter rock, it is espe- 

 cially well developed and often approaches basaltic hornblende in color, 

 birefringence and relief. . 



An attempt was made by the writer to classify the diorites on the basis 

 of their feldspar, into monzonites and diorites. In most of the diorites, 

 the unstriated feldspar constitutes from one third to two thirds of the 

 whole. Williams states that on a number of tests made on the feldspar, 

 however, the specific gravity ran from 2.6-18 to 2.67, which would show 

 it to be plagioclase of the oligoclase-andesine series. Moreover, the re- 

 easting of the subjoined analysis of diorite shows that all of the potash 

 must be in the biotite, although the rock would be called a monzonite, 



25 Amer. Jour. Sci., (3), XXXV, 441. 188& 



