FENNER, THE WATCHVNG BASALT 



1T3 



In hand specimens of natrolite, several features are notable. It is often 

 observed that a finely felted variety changes abruptly to one more coarsely 

 crystalline. The border betrveen the two frequently has a geometrical 

 pattern. There may also be a deposition of hematite dust within the 

 figure outlined, which produces areas of a pinkish color in the midst of 



Fig. 29. Remnants of scolecite needles in the midst of natrolite. 



Slide 143. 



X35. 



solid natrolite. At times, small cavities of similar appearance are lined 

 with projecting natrolite needles. These features are interpreted as 

 attributable to the solution and removal of some preexistent mineral. 

 Generally, this has gone on pari passu with the growth of the natrolite, 

 but in some cases it has been more rapid, leaving cavities for the develop- 

 ment of well-formed terminated crystals. , 



ApophylUte 



The replacement of various minerals by apophyllite in several of the 

 slides has been described (figs. 9, 22, 23). From these relations, but 

 more especially from its relation to calcite, its period of deposition is 

 believed to be among the late members of the zeolite group. In slide 138, 

 it is found in contact with calcite, which appears to be encroaching upon 

 it, but on the other hand, it has been observed in hand specimens appar- 

 ently resting upon calcite. It is probably one of those minerals whose 

 period overlaps that of calcite. 



Ordinarily, a small amount of fluorine is found in apophyllite (on the 

 average about 1.5 per cent). It is possible that in our rocks, this can be 

 ascribed to some preexistent mineral, but it seems more probable that it 

 was derived from a small quantity locked up by the magma when it 



