REPORT OF TEE CHIEF ASTRONOMER 209 



SESSIONAL PAPER No. 25a 



isotropic base is glass. Otherwise the rock is composed of very abundant 

 chlorite and limonite, with some calcite and secondary quartz. The last is 

 always in surprisingly small amount in the base, though the decomposition of 

 the rock is profound. The pores are filled with quartz, chlorite, and opal. 



The non-vesicular zone c is also of a gray-green colour. It is conspicuous 

 by reason of the relatively great size of its abundant feldspar phenocrysts. 

 (Plate 23). These range from one to three centimetres in length, by one to 

 two millimetres in width. In the freshest specimens the phenocrysts have a 

 dull lustre and brownish or greenish colour, both being due to the advanced 

 alteration of the mineral. The feldspar is a plagioclase twinned polysyntheti- 

 cally after the albite law; it proved to be a labradorite near Ab 4 Am.. Under 

 the microscope the crystals were seen to be filled with swarms of minute, 

 secondary foils of sericitic habit but indeterminable (hydrargillite?). These 

 large crystals are embedded in a base which again shows evidence of thorough 

 decomposition, with the formation of much chlorite, much leucoxene, and the 

 same colourless to pale greenish micaceous mineral found in the altered pheno- 

 crysts. In this mass there occur fairly numerous microlites of labradorite 

 (also near Ab 4 An 3 ), one millimetre or less in length. The specific gravities of 

 two of the freshest and most typical specimens are 2 835 and 2-792. Notwith- 

 standing the profound alteration of the rock it was thought that chemical 

 analysis would throw light on its original character. Professor Dittrich has 

 accordingly analyzed the freshest of the collected specimens (No. 1202). It 

 was obtained on the high eastern ridge of the McGillivray range at a point 

 about one mile south of the Boundary line. His results are as follows : — 



Analysis of Purcell Lava (Zone c.) 



Si0 2 41-50 



Ti0 2 3-33 



MA 17-09 



Fe 2 3 3-31 



FeO 10-08 



MnO trace. 



MgO 12-74* 



CaO 0-97* 



Neu,0 2-84 



K 2 0-22 



H,0 at 110°C 0-21 



H 2 above 110°C 6-99 



CO, Done. 



P 2 5 1-08 



100-36 



Sp.gr 2-792 



The analysis evidently does not lend itself to profitable calculation. In 

 spite of the very great alteration, however, the rock is pretty clearly a basalt. 



*A second determination, of CaO gave 0-89 per cent; a third gave 1-01 per cent, 

 with MgO 12-57 per cent. 



25a — vol. ii — 14 



