454 



W. J. MILLER ADIRONDACK ANORTHOSITE 



says, to imagine a batholithic intrusion of the anorthosite followed by a 

 batholithic intrusion of syenite-granite which took the form of a hollow 

 cylinder which invaded the anorthosite only peripherally. This statement 

 by Bowen is opposed by the field facts, since the whole northeastern half 

 of the exposed anorthosite body has been deeply invaded by very consid- 

 erable masses of syenite-granite. 



If we adopt Cnshing's view of Grenville resting on older orthogneiss, 

 this latter rock would have made the country rock distinctly more resist- 

 ant to intrusion by the syenite-granite magma, and hence we should not 

 expect the relative amounts of syenite-granite within the anorthosite area 

 and the region outside of it to be so notably different. So far as I know, 









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t;: jGre/7f///e si rata 



l^jfflffl Pro ba bty.qab b ro or per/dofjfe 

 \^y^^y^Syenife-c^raniie ser/es 



IllllJIIJIJrlgrcy anorfhosife 



^^^:t^^Chilled border of anorfho^'tie Line A B- present eros/on surface. 



Figure 3. — Highly generalized NortJieast-Southwest Structure Section through the 

 Adirondack Anorthosite Body 



Showing the relation of the anorthosite to the Grenville and syenite-granite series 



Gushing does not take up the important problem of the general absence 

 of the syenite-granite series from the anorthosite. 



Strongly supporting my explanation of the absence of the syenite- 

 granite from the southwestern half of the anorthosite area is the evidence 

 from the distribution of the later gabbro stocks. All the more recent 

 workers in Adirondack geology recognize this gabbro as distinctly younger 

 than the syenite-granite series. It occurs usually in the form of stocks 

 or pipelike forms rarely more than a few miles across. Such stocks are 

 very common and widespread throughout the Adirondack region except 

 the anorthosite area. Like the syenite-granite, this gabbro is singularly 

 absent from the southwestern half of the anorthosite body. The detailed 



