Analcite-Trachyte Tuffs and Breccias. 269 



The amount of analcite present in these pyroclastic rocks 

 varies in the different specimens of which there are forty-three 

 in the collection. In the majority of cases it is entirely lacking 

 while inoneit makes upabouttwo-thirds ofthe material. (Fig. I.) 

 When the crystal form has not been destroyed it is seen to be 

 the icositetrahedron. In some specimens the analcite might 

 be mistaken for certain varieties of garnet. It varies from a 

 reddish brown to a rather dirty cream color, the former being 

 due to iron stains. It will be noted in the analysis of this 

 material given below that 2.85% of ferric oxide is found. 

 Under the microscope it is seen to be clear and colorless in 

 irregular patches, the cloudiness being due partly to the iron 

 stains mentioned and partly to minute inclusions — not always 

 determinable — which are sometimes zonally arranged. This is 

 seen to be the case in the crystal in the lower left hand corner 

 of Fig. I. Optical anomalies are not common. Interesting 

 replacements by calcite have taken place in some of the more 

 weathered specimens. This phenomenon is well shown in 

 Figs. II. and III. Some of the smaller analcites show com- 

 plete replacement by albite. This seems to have resulted 

 directly from the breaking up of analcite. 



The properties of analcite, studied in thin section, are 

 such that it is not always possible to distinguish with certainty 

 this mineral from leucite. Both minerals are -isometric, both 

 show optical anomalies ; and inclusions, though more char- 

 acteristic of leucite, are also found in these two minerals. 

 Since, further, the occurence of analcite as a primary mineral 

 in tuffs and breccias has never been described in geological 

 literature up to the present time, it was thought advisable to 

 separate it, by means of heavy solutions, from the other 

 materials and make a quantitative examination. Beforehand, 

 however, it was found to give the characteristic blow-pipe 

 reactions for this zeolite, while small fragments treated with 

 hydrofluosilicic solutions gave the characteristic hexagonal 

 prisms of Na 2 Si F1 B . 



The material selected for analysis is shown in Fig. I. It 

 was taken from a railway cut on the Crows Nest branch of the 

 Canadian Pacific Railway four miles east of Crows Nest lake. 



