REPORT OF THE CHIEF ASTRONOMER 



90 



SESSIONAL PAPER No. 25a 



Minute angular particles of orthoclase or microcline and of microperthite 

 are other, probably original, clastic accessories. A few small shreddy foils of 

 sericitic mica are rare metamorphic constituents. 



The high specific gravity of the dominant phase (2-716 — 2-816) immedi- 

 ately suggests that it is highly magnesian. An analysis of a type specimen 

 (No. 1270) from the box-canyon above mentioned has been made by Professor 

 Dittrich :— 



Analysis of upper Altyn impure dolomite. 



SiO,. 



ALO, 



Fe^O', 



FeO 



MgO 



CaO 



Xa„0 



K„0 



H,0 at 110°C. . 

 PLO above 110°C, 



co 2 ... 



Mol. 



26-07 



435 



3-92 



•038 



2-08 



•013, 



2-68 



•037 



12-99 



•325 



19-58 



•349 



1-04 



•017 



1-40 



•015 



•04 





1-52 



083 



29-14 



662 



Sp-gr 



Insoluble in hydrochloric acid. 

 Soluble in hydrochloric acid: 

 Fe,0, 



ai;o; 



CaO 



MgO , 



100-46 

 2-816 



30-80% 



4-25 

 2-17 



19-38 

 12-69 



In this case, the relative quantities of the different components may be 

 roughly calculated. The proportions are approximately as follows : — 



Calcium carbonate 34-6 



Magnesium carbonate ' 26-7 



Free silica 15-0 



Albite molecule 9-0 



Soricite and potash feldspar, about 9-7 



Remainder (pyrite, limonite, etc.) about 5-0 



1000 



It is, however, possible that the mica bears some of the soda and that 

 paragonite itself may be present. If so, the foregoing calculation would be 

 manifestly incorrect. In any case the carbonates compose rather more than 

 60 per cent of the rock. The ratio CaO : MgO is 1-527:1 and thus not far from 

 the theoretical ratio in pure dolomite. In all essential respects this sediment 

 is like that of the type of Upper Altyn in the Clarke range except in the higher 

 proportion of quartz and silicate material in the Galton series type. In each 

 case the rock is a silicious dolomite. 



Hefty Formation. 



In the MacDonald range the Altyn formation is conformably overlain by 

 a group of strata which are exposed at the same two localities where the Altyn 

 25a — vol. ii — 7i 



