ALTERATION OF THE FELDSPARS. 1G1 



the optical orientation, even to the minutest twinned lamellae, is iden- 

 tical on either side of the line of sericite, and the feldspar appears very 

 fresh as compared with that not thus cleared of its inclusions. Its polari- 

 zation colors are much more uniform in the cleared areas, and the greater 

 the amount of the impurities and the more unequal their distribution, 

 the greater the variety of mottled, tinted colors under crossed nicols. In 

 case there are two lines of sericite close together, the interspace may he 

 entirely cleared (see a', figure 5). 



In attempting to interpret correctly the cause of the outer clear rim 

 about feldspar pebbles only two hypotheses need be considered — it is 

 either secondarily deposited feldspar or is produced by some subsequent 

 action that eliminated the interpositions. If secondary, then the outlines 

 of the feldspar areas having inclusions are too jagged, and concave sur- 

 faces are too common to have been the result of ordinary attrition, while 

 the possibility that the new feldspar was added so as to change their out- 

 line to that of normal pebbles is very improbable. In the case of the 

 largest pebble in the section glassy feldspar accompanied by sericite 

 folia follows the outer part of the pebble for a distance, then penetrate 

 into the interior, following lines of sericite. Bordering areas of the back- 

 ground not containing sericite have no complementary clear zone in the 

 feldspar. Interior lines of sericite and their accompanying dependent 

 zones are only explainable in one way. To assume that they are sec- 

 ondary growths necessitates the subsequent displacement of the feldspar 

 clastic a distance equal to the cleared areas and a consequent probable 

 displacement of its optical continuity. As a matter of fact, all parts extin- 

 guish as a unit, the conformity of the positions of the twinning lamellae 

 on either side of the serecite indicating their original crystallographic 

 and optical continuity. Examples of apparent secondary enlargements 

 of clastic feldspar are becoming so commonly described that it may be 

 well in the future to bear in mind this phenomenon, which simulates so 

 closely that of genuine enlargement and which may have led to misin- 

 terpretations in the past. 



Alteration of clastic Microcline into Plagloclase. — In connection with the 

 study of secondary feldspars in metamorphic rocks, it is interesting to 

 note the trend of certain phenomena observable in the granulated end 

 of this large pebble mentioned above where secondary albite and clastic 

 microcline are intermingled. Dr Wolff has lately called attention to a 

 possible relationship between detrital areas of feldspar and secondary 

 albites,* and the facts here observed substantiate his interrogative hy- 

 pothesis. With but very few exceptions in the Vermont rocks studied 

 by me have I noticed secondary feldspars free from inclusions of seri- 



*Metamorphism of Clastic Feldspar in Conglomerate Schist: Bull. Mus. Comp. Zool., \<>l xvi, 



no. id, p. ls:i. 



