REPORT OF TEE CHIEF ASTRONOMER 229 



SESSIONAL PAPER No. 25a 



common granites. The chemical analysis of the rock shows the mica to he 

 magnesian. 



A striking feature of this, as of the other phases of the acid rock, is the 

 advanced alteration of the feldspars which are usually filled with dust-like 

 aggregates of epidote, kaolin and muscovite. This alteration is believed to be 

 due to magmatic after-action, probably the result of the expulsion of vapours 

 during the solidification of the underlying gabbro. 



The calculation of the quantitative mineralogical composition of the rock 

 has been attempted by the Rosiwal method. In the process the secondary pro- 

 ducts were neglected and the feldspars were arbitrarily regarded as fresh. The 

 inaccuracy of the result is manifest but it doe9 not affect the value of the com- 

 parison among all the phases of the sill. Especially between the gabbro and 

 the acid zone the contrasts of quality emerge with the same clearness and 

 certainty as characterize the related contrasts established in the chemical 

 analyses. 



The total chemical analysis by Prof. Dittrich of this Phase 1 (specimen No. 

 1137) is here given: 



Analysis of Granite (Phas-e 1) of Moyie Sills. 



MoL 



Si0 2 71-69 1-195 



TiO, -59 -005 



ALA 13-29 -130 



Fe-A -83 -005 



FeO , 4-23 -058 



MnO » -09 -001 



MgO 1-28 -032 



. CaO 1-66 -030 



Na,0 248 -040 



K.0 2-37 -025 



H,0 at 110° C -14 



H 2 above 110°C 1-31 



R0 5 -07 



C0 2 -13 



100-16 



Sp.gr (corrected value). 2»733 



This rock is clearly an unusual type of biotite granite. The most evident 

 peculiarity is the low total for the alkalies; it accords with the relatively small 

 proportion of feldspar present. Notwithstanding the abundance of free quartz, 

 the silica percentage is kept low by the comparative richness in biotite and by 

 the magmatic alteration of the rock. The estimate of the mineralogical com- 

 position gave the following result in weight percentages : — 



Quartz 41-6 



Sodiferous orthoclase 32-5 



Biotite 15-2 



Muscovite * 4-6 



Microperthite 3-9 



Oligoclase l - 



Magnetite 1*0 



Apatite *2 



loo-e 



