REPORT OF THE CHIEF ASTRONOMER 399 



SESSIONAL PAPER No. 25a 



minerals. The specific gravities of four compact, comparatively fresh specimens 

 vary from 2-663 to 2-759, thus roughly indicating the variable proportion of 

 glass in the rock. At times this lava is highly scoriaceous; calcite and chlorite 

 usually fill the gas-pores. 



Augite andesite is yet more abundant than the basalt. In many essential 

 respects excepting in the absence of phenocrystic olivine and in the lower per- 

 centage of the pyroxene and iron oxide, this andesite seems to be lithologically 

 similar to the basalt. One fresh compact specimen has the specific gravity, 

 2-733. Like the basalt and, in fact, like all the other Midway lavas, the ande- 

 site is not crushed or sheared but it is considerably altered as if by ordinary 

 weathering. 



The hornblende-hearing andesites were found at only one locality. The 

 top of the low mountain immediately to the northwest of the confluence of 

 Myer's creek and the river, is composed of a vesicular to compact andesite 

 which bears phenocrysts of labradorite, hornblende, less abundant augite, and 

 sometimes biotite. The biotite-free phases seem to be the commonest in the 

 sections actually traversed. The sporadic appearance of the mica among the 

 phenocrysts is a phenomenon not well understood but it was observed that, in 

 one thick flow finely exposed in the cliff overlooking the river, the compact 

 interior of the mass carried all three femic phenocrysts, while the vesicular 

 surface shell of the flow carried only hornblende and augite. 



Macroscopically these two andesites are very similar, with a fairly light, 

 greenish-gray to brownish-gray colour except where the lustrous-black pheno- 

 crysts interrupt the general surface of the rock. In each case the ground-mass 

 is habitually hypocrystalline or else a microcrystalline aggregate of andesine, 

 hornblende, rare augite granules, and considerable primary quartz. 



In the precipitous cliff just mentioned the hornblende-bearing andesites 

 are seen to overlie vesicular flows of hiotite-augite andesite. The vesicles of 

 these conformable flows are here and there arranged in rude layers which indi- 

 cate a probable local strike of the series of 1ST. 30° E. and a dip of 10°-20° to 

 the northwestward. 



The augite-biotite andesite is much darker coloured and clearly more basic 

 than the overlying flows. Labradorite is a never-failing phenocryst. The 

 biotite is sometimes more abundant than the augite but the reverse relation 

 often holds. The ground-mass is generally glassy, with microlites of feldspar 

 and augite. Primary quartz seems to be quite absent. The specific gravity 

 of a non-vesicular, fresh specimen is 2-633, showing the presence of consider- 

 able glass. As usual with these lavas both phenocrysts and ground-mass are 

 often much altered. The gas-pores of the vesicular parts are filled with quartz, 

 calcite, or a yellow zeolite, probably delessite. In two thin sections of this 

 andesite the base was found to carry orthoclase in considerable amount. It 

 occurs in minute, allotriomorphic individuals which are intersertally arranged 

 with respect to the ground-mass microlites of andesine. The abundance of 

 orthoclase and the presence of biotite suggest a content of potash like that of 

 some latites. 



