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Canadian Record oj Science. 



that in certain cases this so called glassy ground mass was 

 really analcite, which was the final product of crystallization. 

 As has been pointed out by Prof. Iddings, the difficulty of dis- 

 tinguishing an isotropic substance like analcite from residual 

 glass is at once apparent. 1 Dr. Coleman has described a dike, 

 closely related to the tinguaites, from the north shore of Lake 

 Superior, which is exceedingly rich in analcite. 2 It contains 

 in some cases as much as 47% of this zeolite. One of the 

 crystal tuffs from Alberta, however, will run even higher than 

 this. Fig. I. is a photo-micrograph of a specimen in which it 

 will be seen that the analcite will run over 60% of the rock. 

 It is practically an analcite-tuff. 



Fig. 2. Analcite replaced by calcite ; crossed nicols ; actual field is 2-5 mm. 

 The large black crystal with octagonal outline, near the centre of the field, is 

 analcite ; it shows no replacement. Immediately to the left is another analcite 

 with hexagonal outline showing replacement by calcite (white) around the 

 edges and ramifying into the centre. 



The reasons for believing that analcite is a primary mineral 

 of these tuffs are given below. The word primary means, as 

 already stated, that the analcite has been deposited from 



1. Jour. Geo., Vol. I., p. 638. 



2. Ont. Bur. of Mines :£a8, p. 172. Also Rep for 1S99, p. 186. 



