PREFACE 
The writer’s experience in the Adirondack Mountains of New 
York state has led him to realize the lack of published information 
concerning the perthitic feldspars. The dominant feldspar of the 
Algoman augite syenite of the region is perthite (microperthite). 
This Precambrian rock is a differential phase of an important rock 
unit which grades from syenites, through quartz syenites, and syenite 
granites to true granites. In passing from the femic to the salic 
phases it is readily observed that the feldspar becomes more 
potassic and passes into what is commonly known as orthoclase, 
and thence into microcline. Thus the Algoman syenite-granite 
rocks raise two questions: What is perthite? And what is the 
difference between orthoclase and microcline ? 
The attempt to answer these and similar questions led the 
writer to make the investigation of which the following paper is 
the result. 
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