FENNER, THE WATCHVNG BASALT 



141 



acteristics. Their relations among themselves and to the associated 

 minerals present several interesting features. 



Variety 1. In a hand specimen of pectolite, altered in part to crystal- 

 line calcite, small, tabular crystals of some mineral of a dark-green color 

 were noted. A thin section (116) disclosed the relations shown in fig. 14. 

 The dark mineral is surrounded by calcite, and between the two there is 

 often a muddy border, indicative of decomposition. The process of re- 

 placement has frequently left the curious projecting shreds shown in the 

 figure. The general habit and the optical properties indicate an amphi- 

 bole, but several features are abnormal. 



The pleochroism is strong. X = dark blue-green, Y = claret and 

 Z = pale yellow-green. Intermediate sections show tints of gray, lilac. 



Fig. 14. Shreds of an abnormal amphibole (probably arfvedsonite) in calcite. 



X 32. Slide 116. 



purple and brown. Absorption formula, X > Y > Z. There is a well- 

 defined cleavage, probably parallel to the prism, and probably another 

 cleavage whose character could not be determined. Elongation tabular 

 parallel to the prism. Optic character biaxial and negative, optic angle 

 large. Extinction measured from the X-axis of elasticity to what is 

 probably the vertical cleavage makes a maximum angle of 14-15°. 



Unusual features are the reversal of the ordinary formula of absorp- 

 tion for amphiboles, the position of the X-axis, and the great range of 

 pleochroism. These features agree very closely, however, with the rare 

 variety arfvedsonite (Na^, Ca, Fe)4 Si40i2 with (Ca, Mg)^ (Al, Fe)4 

 SigOjo, as described by A. X. "Winchell.*^ In the same slide, a small de- 



" "Optical Mineralogy," p. 114, 1009. 



