FENNER, THE WATCHUNG BASALT 143 



corresponding very closely to that of associated prehnite, and indicating a 

 valne of birefringence of about 0.033. Its affinities seem to be with actin- 

 olite. Its strong pleochrism points to a high content of ferrous iron.** 



In 53, green amphibole occurs in associations very like those in 98. 

 Plate XI, fig. 5, shows a portion of the slide, but not that in which the 

 amphibole is best developed. The pleochroic colors are the same, and the 

 general appearance is very similar, but the habit is more columnar, and 

 the plumose development is much less prominent. Extinction angles 

 measured from the elongation gave maximum angles up to 32 or 33° (in 

 a doubtful case still higher). Pleochroism, emerald-green parallel with Z, 

 pale yellow and brown parallel with X and Y. Absorption corresponds. 

 The elongation makes an acute angle with the vibration direction Z. The 

 birefringence is practically that of prehnite, or about 0.033. Index of 

 refraction greater than that of calcite (> 1.66). The high birefringence 

 may indicate an approach to the iron amphibole, griinerite, but the 

 large extinction angle and character of the pleochroism are somewhat 

 anomalous. 



The relation of the amphibole and garnet in this slide is at variance 

 with what was found in 98 (described under garnet). In the latter, the 

 amphibole held inclusions of the garnet and was quite plainly replacing 

 it. In 53, it is even more evident that garnet occupies areas formerly held 

 by the amphibole. The replacing garnet is transparent ( free from opaque 

 dust), so that the forms of the former amphibole prisms are distinctly 

 outlined. In some instances, the replacement is not complete and rem- 

 nants of amphibole are still left. 



In 109, from the same hand specimen as 53, there appears to be the 

 following relation of garnet and amphibole (Plate XIII, fig. 2). The 

 stouter prisms of amphibole have been partly replaced by garnet. This 

 has not proceeded very far, and a reversal of the process has occurred by 

 which a bordering rim of fil)rous amphibole has grown out among the 

 garnet grains, partly replacing them. At the same time, the stouter 

 prisms themselves have broken up into the fibrous form, which preserves 

 fairly well the outward boundary but has assumed an internal structure 

 which is even plumose in places, without much regard to the original 

 crystallographic orientation. In the middle portion of the drawing, a 

 single })rism of amphibole illustrates both processes. At one end, the 

 amphibole has been replaced by colorless garnet. Toward the other end, 

 fibres of amphibole have shot out among the garnet grains for considerable 

 distances beyond the original border of the rod. The structure of the 



" Dana : Op. cit.. p. 389. 

 WiNCHELL : Op. cit., p. 109. 



