TRANSITION ROCKS 445 



for fewer labradorites, and I believe it to be Keene gneiss with the same 

 history as that on Cobble Hill. 



On much larger scales I have mapped two areas of Keene gneiss, one 

 occupying about 6 square miles and the other nearly 3 square miles. Be- 

 fore the intrusion of a large gabbro stock the two areas were probably 

 connected with a total length of nearly 7 miles. These bodies of Keene 

 gneiss lie against typical Marcy anorthosite, the border facies of anor- 

 thosite here having been very largely assimilated by granite or syenite- 

 granite mdgma. Throughout the larger area especially there are a good 

 many small masses of Whiteface anorthosite, a few of sufficient size to be 

 mapped, and some outcrops of fairly good granite or granitic syenite, 

 thus showing that the assimilation was not everywhere thorough. The 

 main bodies of the rock are quite typical Keene gneiss. The inclusions 

 of anorthosite indicate the true intrusive character of the Keene gneiss. 



Enough examples have been described to prove that Keene gneiss has 

 developed on small and large scales by assimilation of anorthosite by 

 granite or granitic syenite magma instead of by syenite magma, as is 

 usually the case in the other Adirondack districts thus far studied. If 

 we adopt Bowen's hypothesis, this Keene gneiss must be regarded as hav- 

 ing developed by differentiation in situ between an overlying sheet of 

 syenite-granite and underlying anorthosite. If one admits, as I do not, 

 that syenite usually may have developed by differentiation in situ close 

 on the Marcy anorthosite, how can one imagine, in other places like the 

 Schroon Lake quadrangle, a similar development of granite close on the 

 anorthosite? It might be argued that the granite magma formed at a 

 higher level and was then forced downward. But, if so, it must have been 

 forced downward through the still lower syenitic material. Not only is 

 the field evidence against this view, as already pointed out, but even if 

 we grant it, we are still forced to the conclusion, by the obvious field facts, 

 that the granite magma produced the transition rock (Keene gneiss) by 

 assimilation of more or less anorthosite, and that the Keene gneiss was 

 not formed as a differentiate in situ between an overlying sheet of syenite- 

 granite magma and underlying anorthosite. 



KEENE ONEISS OF OTHER ADIRONDACK REGIONS 



In Cushing's report on the geology of the Long Lake quadrangle, he 

 describes a basic phase of the syenite which grades into a rather fine 

 grained, even granular, gneissoid rock with few feldspar phenocrysts and 

 dark minerals often equaling or exceeding the feldspar in quantity. Some 

 of the feldspar is microperthite and some oligoclase-andesine. 



