572 MacKenzie — Analcite of the Crowsnest Volcanics. 



indicates no reason to suppose conditions favorable for the con- 

 version of leucite to analcite to have been present. 



(2) Supposing, however, that the pyroclastics were deposited 

 in salt water, we may critically inquire into the possibility 

 that the present analcite represents converted leucite. 



Lemberg's experiments* clearly show that finely-powdered 

 leucite is converted into analcite by treatment with solutions 

 of sodium salts. This reaction, it is indicated, may take place 

 even with rather dilute solutions and at relatively low tempera- 

 tures. While these experiments are significant, the extent to 

 which they may be called upon to explain analcite occurrences 

 in igneous rocks is problematical. 



In the case under consideration the analcite phenocrysts are 

 euhedral trapezohedrons up to one inch in diameter, and form 

 50 per cent of the volume of the rock. Fifty per cent of the 

 remainder are composed of smaller analcite phenocrysts and 

 analcite in the groundmass. The analysis of the phenocrysts 

 shows that the analcite is in a high state of purity. 



The large trapezohedrons in thin section are quite homogene- 

 ous, with the analcite cubic cleavage well developed, proving 

 that they are single individuals. That this condition resulted 

 from a conversion of leucite to analcite seems inconceivable. 

 Lemberg's experiments, carried out on finely-powdered mate- 

 rial, throw no light on the mechanism of the change. Whether 

 it is a molecular exchange, or merely a substitution of sodium 

 for potassium atoms with the taking on of a molecule of water, 

 we can only surmise. In either case, the conversion would 

 proceed from the exterior of the crystals inward, and it is alto- 

 gether unlikely that the conversion would have been so com- 

 plete in every case as to leave no trace of the original leucite. 

 Supposing a replacement by equal volumes to have taken place, 

 it is probable that a granular aggregate of analcite would 

 replace the original leucite, a tendency which would be accentu- 

 ated by the fact that leucite generally is composed of twinned 

 layers. 



If, instead of replacement by equal volumes, the molecules 

 of analcite be supposed to take the place and orientation of the 

 original leucite molecules, an increase of volume of about 10 

 per cent would result, with accompanying distortion, an effect 

 that has not been observed. 



The analcite-bearing rocks are, as described,f very slightly 

 altered. This is a strong argument for the primary character 

 of the analcite, and the writer would refer to Pirsson's well- 

 stated argument in maintaining the primary nature of the anal- 

 cite in some of the basalts from the Highwood mountains.^ 



*Zeitsohrift der deutschen Geol. Gesellsch, xxviii, pp. 535, et seq., 1876. 



t Op. cit., p. 28. 



% Jour. Geology, vol. iv, p. 686, 1896. 



