GEOLOGY OF MOUNT MARCY 47 



COMMENTS BY J. F. KEMP 



It will be noted that in one of the specimens studied by Mr Roesler, . 

 minerals suggesting an aplitic addition to the anorthosite from an 

 outside source were observed. Some apparent corroboration of the 

 suggestion was gained from a specimen of Canadian anorthosite with 

 an aplite dike, in the collection of Professor Pirsson. Yet all the 

 specimens furnished Mr Roesler by the writer came from one large 

 boulder of otherwise normal anorthosite, and no intrusive dike of 

 any sort appeared in it nor have we yet observed aplite intrusive 

 in the anorthosites/ Pegmatites of granitic composition are known 

 in a few cases, but nothing as yet that would be described as aplite. 

 The aplitic minerals would appear to be necessarily due to some 

 reaction in the anorthosite itself. With regard to the formation of 

 the reaction rims it may be further noted that they appear around 

 large and thoroughly uncrushed crystals of labradorite, caught in the 

 great masses of titaniferous magnetite on Lake Sanford, in the 

 neighboring Santanoni quadrangle; and that they are observable 

 sometimes in the basic gabbros which are still not appreciably granu- 

 lated. They may in instances, at least, be due to some magmatic 

 corrosion of older formed minerals, and in the closing stages of 

 crystallization. Nevertheless the garnets in the thoroughly mashed 

 anorthosite would seem to be the results of some reaction of pyrox- 

 ene and labradorite, incident to the dynamic metamorphism because 

 they are sometimes from half an inch to an inch in diameter, not 

 apparently granulated, and looking in the crushed anorthosite much 

 like a red knot in a pine board. 



Peculiar gabbro. In one of the earlier season's work a ledge was 

 observed which had been blasted shortly before in highway improve- 

 ment and which proved to be a very curious departure from the 

 normal anorthosite. The locality is on the old road from Keene 

 valley to the Ausable Club (Beede on the map), and within one- 

 fourth of a mile of the forks. In recent years a new road has been 

 built farther west. The rock impresses one as a gabbro-porphyry. 

 Large, rectangular pale-green crystals of labradorite are set in a 

 blackish green finer grained matrix. Under the microscope the 

 dark-green ground mass is resolved into granular, green augite. 

 Some augites are larger than others, but as a whole granulation is 



1 Since the above was written H. L. Ailing has found a narrow aplite dike 

 several inches in width cutting an anorthosite boulder on East hill, northeast 

 corner of the quadrangle. Under the microscope the essential minerals are 

 quartz, soda-microcline and microcline-mioroperthite. The accessory minerals 

 are magnetite, uralite, biotite and diopside. There is considerable garnet in 

 the rock which is regarded as resulting from metamorphic action. 



