FENNER, THE WATCHUNG BASALT 



171 



hematite, while the areas thus outlined have the form of chabazite, al- 

 though wholly occupied by interlocking natrolite crystals. In 94 also, 

 natrolite is seen to be advancing across chabazite. 



Several hand specimens show a distinctive method of replacement of 

 chabazite by laumontite. The laumontite appears to start from some 

 nucleus within a solid crystal of chabazite and grow in radial crystals, 

 until there may be only a crust of chabazite left. Slide 147 contains 

 several radial groups of this kind. In 77 also, there is replacement of 

 chabazite by laumontite, though in neither of the thin sections does the 

 evidence appear as decisive as in large specimens. 



Fig. 28. Needles of natrolite which have penetrated areas of stilbite. 



Slide 121. 



X 35. 



In 120 and 138, there is an association of laumontite and heulandite. 

 The relations are not as clear as might be desired, but laumontite appears 

 later. 



Replacement of all three minerals by caleite is seen in numerous in- 

 stances. It appears that chabazite, heulandite and stilbite followed each 

 other in this order and that they were all earlier than natrolite, caleite 

 and laumontite. 



Laumontite 



Because of the friable nature of laumontite, it was found difficult to get 

 ^ood microscopic sections showing it. There are only a few, therefore, 

 which illustrate its occurrence, and in these the relations rarely appear 

 of a decisive nature. On the other hand, fortunately, it is one of the few 

 minerals which occur in the large specimens in such a manner as to make 

 the sequence beyond question. It is frequently seen resting upon drusy 



