224 



University of California. 



[Vol. i. 



being bordered on either side by the blue. The pleochroism of the 

 inner strip in such cases is light greenish yellow to colorless trans- 

 verse to the prism, and light green parallel with it. Its angle of 

 extinction is also greater than that of the blue amphibole, often being 

 as great as 20 , while the latter seldom exceeds 6°. By no means 

 all of the crystals are so zoned; many are composed throughout of 

 the blue variety, although never of the green. A parting of the 

 long prisms transverse to the longer axis is very common, and the 

 different portions have frequently been displaced and the intervening 

 spaces filled in with the crystalline groundmass. In such cases the 

 broken ends of the prismatic sections are often tipped with the blue 

 amphibole in a manner indicative of replacement rather than second- 

 ary growth or enlargement. 



The blue amphibole, so conspicuous in this schist, is apparently 

 glaucophane, as in the case of the schist adjacent to the fourchite. 

 The mica plate shows that the axis of least elasticity corresponds 

 with the crystallographic c, and the angle of extinction does not 

 exceed 7 . In other slides, however, larger angles have occasion- 

 ally been noted, and it is quite possible that more than one species 

 of blue amphibole occurs on the island. In the case of the serpen- 

 tine contact zone, the mineral doesnot occur in a condition so favor- 

 able for investigation as it does next to the fourchite sill, and its 

 specific determination in every slide would be a matter of consider- 

 able difficult}', if not an impossibility. The green amphibole may 

 be simply classed as actinolite. 



Schist, generally similar to that just described, borders also the 

 western edge of the large southern division of the dyke, and accom- 

 panies the smaller exposures along the shore to the west. Near 

 the most northerly of these three smaller areas, the schist has a 

 decidedly knotty, or conglomeritic, appearance. The whole is now 

 perfectly recrystallized, but the "pebbles" are harder and more dense 

 than the matrix, and stand out on weathered surfaces. If, as is 

 barely possible, these were originally water-worn pebbles, they have 

 been squeezed and flattened out of their former shape. In general 

 they can be knocked out of the matrix, and the latter is usually 

 more rich in brown mica immediately surrounding them than else- 

 where. 



