28 NEW YORK STATE MUSEUM 



Chemical considerations and field facts both indicate that this latter 

 question is probably to be answered in the negative. The chem- 

 ical considerations bring out the improbability of the formation 

 of anorthosite in any manner other than by the accumulation of 

 plagioclase crystals precipitated from solution in a mixed magma." 

 He therefore conceives of " the Adirondack anorthosite-syenite 

 complex as essentially a stratified mass with syenite above and 

 anorthosite below." ^ 



Bowen's arguments from the theoretical standpoint, at least, are 

 not to be lightly brushed aside. But after months of detailed field 

 work in a region like the Lake Placid quadrangle where anorthosite 

 is so prominently developed in all its phases, the writer believes 

 that certain field facts are most decidedly opposed to Bowen's 

 conception.^ The main facts of this sort will be here briefly men- 

 tioned, actual examples and detailed descriptions being given else- 

 where in this bulletin. 



1 Sharply defined inclusions (small and large) of Grenville rocks 

 occur in many portions of the anorthosite. These certainly bear 

 every evidence of having been enveloped in an active magma. 

 According to Bowen's hypothesis, how can such inclusions be 

 accounted for? 



2 In a number of places, clearly defined inclusions of anorthosite 

 have been found in the syenite-granite series. Could fragments of 

 the anorthosite, if formed by the settling of plagioclase crystals, 

 have been forced upward by some process into the syenite-granite 

 magma? Is it not much more plausible to regard these inclusions 

 as indicating the envelopment of previously solidified anorthosite 

 in an active syenite-granite magma? 



3 Tongues or dikes of syenite and granite, as off-shoots of large 

 masses of similar rocks, are known definitely to cut the anorthosite. 

 Must we assume that tonguelike masses of overlying molten syenite 

 or granite were forced downward into the anorthosite? 



4 In a number of areas there is clear evidence that anorthosite 

 has not only cut to pieces, but also intimately injected, Grenville 

 gneiss. Could such injection gneisses have developed except by 

 forcible and intimate intrusion of a highly molten mass into the 

 Grenville ? 



^ These quotations are from an abstract o£ Bowen's paper in Bui. Geol. 

 Soc. Amer. 28:154. 1917. 



2 The interested reader should consult Bowen's paper " The Problem of 

 the Anorthosites " in Jour. Geol. 25:209-43, 1917, and the writer's paper 

 "Adirondack Anorthosite" in Geol. Soc. Amer. Bui. 29:399-462, 1918. 



