PERTHITIC FELDSPARS. 129 



potash predominating. The Bedford feldspar has apparently a simi- 

 lar relationship, though little is known about it; that from Perth is 

 probably connected with some of the soda-rich magmas of Ontario. 



Summary Description of the Structural Characteristics of the Feldspars 

 Studied. — The microscopic characteristics of the perthitic feldspars are 

 so familiar to mineralogists and petrographers and have been so often 

 and so fully descriljed, that it seems unnecessary to give a detailed 

 description of the samples here studied. A few general observations 

 will be made and a brief summary of some of their individual char- 

 acteristics will be given. 



The general form of the albite member of the intergrowth is most 

 commonly that of relatively thin lamellae following a direction approx- 

 imately parallel to the crystal plane, 801. Other directions are some- 

 times followed, nor does the albite always form lamellae, but rather 

 more or less flattened and irregular prismoid bodies. The crystal 

 structure of the two members is, however, always substantially 

 parallel. 



In sections parallel to the base (001) the outcrop of the lamellae is 

 in general roughly parallel to the edge 001-100. Taken in detail 

 there are many minor curvings and changes of direction, and often 

 very marked deviations may be noted. Perhaps the most common 

 one is in a direction that appears to be in a direction parallel to that 

 of a imit prism. Sometimes a single band will curve gradually around 

 in this direction ; again a number of short lamellae will coalesce along 

 the same direction. In a recent paper A. F. Rogers ^ has called atten- 

 tion to intergrowths showing this characteristic, citing particularly a 

 perthite from near Port Henry, N. Y., but referring also to that from 

 Perth, Ont. The present writer has noted perthite with this feature 

 developed in a quite striking manner in specimens from Haddam 

 Neck, Conn., on which it was originally intended to carry out quan- 

 titative studies. There is a great variation in width in different 

 samples and in the same sample. A single lamellae will also gen- 

 erally show much variation in this respect. They rarely exceed 2 

 mm. in width in any specimen and not very often 1 mm. The 

 usual width is much less. The length of the bands also varies greatly 

 even in the same specimen. They frequently pinch down and swell 

 again, or pinch out entirely; branching and anastomosing are exceed- 

 ingly common. 



In sections parallel to the bracy-pinacoid the lamellae make an angle 



3 Jour. Geo!., 21, 5 (1913). 



