134 WARREN. 



clear in color; the albite lamellae are very clearly marked. They 

 rarely exceed 0.6 mm. in width. The bands follow fairly well the 

 usual direction although they frequently bend away from it. This 

 is true also in the 010 sections where there is a rather unusual amount 

 of departure from the usual angle of about 70° with the basal clea\'- 

 age. A few independent albite crystals were noted. Both feldspars 

 are strongly twinned. The structure of the microcline as seen in 010 

 sections is peculiar. Between crossed nicols the extinction is not 

 uniform. There seems to be a sort of a fine lamellar structure devel- 

 oped with varying distinctness in different areas. This is often more 

 distinct against a cleavage line where minute wedge-shaped lamellae, 

 having different positions of extinction, are to be seen. The extinction 

 angles vary from 1° to 7° but are never sharp. The impression is that 

 there has been a slight deorientation of the feldspar following particu- 

 larly along the general direction of the intergrowth which is sub- 

 stantially that of the Murchinsonite parting. It is probably here 

 also connected with the slight variation in level observed in the 

 main cleavage surfaces and has probably been caused by crushing 

 movements in the pegmatite subsequent to its consolidation. The 

 twinning in the albite shows also some irregularities of position. 

 Extinction angles are: — • 



Microcline — ^ on 001 .... 17° 



— " 010.... l°to7°. 



Albite — " 001 ... . 3° 



— " 010.... 17° 



Quantitative Methods of Sfudy employed; Microscopic. — The relative 

 amounts of the two members of the intergrowths were determined by 

 the well knowTi micrometric method first described by Rosival. From 

 a carefully chosen clea\'age fragment of each feldspar to be studied, 

 two very large thin sections were cut, one parallel to the basal, and the 

 other parallel to the brachypinacoidal cleavage. These sections were 

 especially prepared for the writer by the firm of Voigt & Hochgesang, 

 of Gottingen, Germany, and no little credit is due to them for the 

 exceptional quality and size of the sections. The measurements were 

 all made with the large mechanical stage of Zeiss, which is superior to 

 any other with which the author is familiar for measurements of this 

 kind. The scales used were all carefully calibrated. In general the 

 lines of measurement were run at distances of a millimeter apart. 



