REPORT OF THE CHIEF ASTROXOMER 79 



SESSIONAL PAPER No. 25a 



Portion insoluble in nydrochloric acid 32-23% 



Portion soluble in hydrochloric acid : 



Pe 2 O s 2-36 



AL0 3 2-29 



CaO 18-86 



MgO 12-96 



The carbonates could be rather closely calculated if it were known 

 how much of the ferrous iron is present in the sideritic molecule. Since the 

 carbon dioxide is no more than sufficient to satisfy the lime and magnesia of 

 the soluble portion, it is probable that iron carbonate is present in but very 

 small amount. A definite assignment of the alkalies is here impossible. The 

 soda is arbitrarily assigned to the albite molecule, although it is possible that 

 paragonite is present. A partial calculation yields the following result: 



Calcium carbonate 33-7 



Magnesium carbonate 27-2 



Albite molecule 5-2 



1'ree silica 120+ 



Sericite, potash feldspar, iron oxide, etc 21-9- 



100-0 



The ratio of CaO to MgO in the soluble portion is 1-455:1, a value very 

 close to that in normal dolomite. 



Dolomite forms about 61 per cent of this specimen. Probably nowhere 

 in the formation does it form more than 75 per cent of any bed. The percent- 

 age in the more silicious beds may run far below 40 per cent, as shown by the 

 specific gravity, 2-630, of one fresh specimen. 



Inasmuch as the mineral dolomite dominates over the free quartz, the 

 staple rock of the formation may be classified as a highly silicious dolomite. 

 Chemically this rock notably resembles the Siyeh magnesian limestone and the 

 Altyn dolomite. Also important are the similarities of structure and size of 

 grain, implying like conditions of origin for the essential carbonates. 



Intertedded Lava. — The amygdaloidal lava bed near the base of the forma- 

 tion is similar in composition to the Purcell Lava and doubtless represents a 

 local, somewhat later flow from the same basaltic magma. This lava is vesicular 

 throughout, much more so than the Purcell Lava. The vesicles are, as usual, 

 particularly large, and numerous near the upper surface of the flow. The larger 

 ones approach 1 cm. in diameter. In the highly vesicular phase the vesicles 

 average about 2 mm. in diameter and compose from one-quarter to one-third 

 of the rock's whole volume. The vesicles are generally completely filled with 

 well crystallized calcite, less often with granular or radially crystallized quartz, 

 and a few are filled with both calcite and quartz. There is nothing specially 

 noteworthy concerning the silicious amygdules, but the large majority of the 

 pure-calcite amygdules present a remarkable phenomenon which, so far as 

 known to the writer, has not been described in petrographic literature. 



In the highly vesicular p^ase of the lava the calcite of many associated 

 amygdules is all rigidly ori r tated so that there is simultaneous reflection of 

 light from a cleavage-surf:. ■. -'■; in each exposed amygdule. Careful examination 



