262 DEPARTMENT OF T\HE INTERIOR 



2 GEORGE V., A. 1312 



by another band of dolomite, which is very similar to the first dolomite occur- 

 ring at the western, end of the section. The dolomite here is about 450 feet 

 thick. 



The specimen of this dolomite (No. 886) is fairly typical not only of the 

 whole band but also of the whole group of carbonate bands occurring in the 

 Priest River terrane. It has, accordingly, been selected for chemical analysis. 

 On the fresh fracture the rock varies in colour from white to pale blue and 

 weathers rather uniformly brownish yellow or buff. It is transected' by numer- 

 ous veinlets of white quartz and by others of very compact dolomite. Other- 

 wise the roek is a very homogeneous, fine-grained, marble-like mass of carbonate, 

 which in the ledge shows no appreciable impurity. The specific gravity is 

 2-822, corresponding to normal dolomite pretty closely. 



The analysis by Professor Dittrich, afforded the following result : — 



Analysis of dolomite, Priest River terrane. 



Mol. 



SiO, 5-84 -097 



5 ALA -80 -008 



FeA -79 -005 



FeO .16 -002 



MgO 19-38 -485 



CaO 28-31 -506 



Na 2 0' A -27 -004 



K,0 -09 -001 



H 2 at 110°C -03 



H 2 above 110°C -63 -035 



C0 2 43-55 -990 



99-85 



Sp.gr 2-822 



Portion insoluble in hydrochloric acid, 5-96% 



Under the microscope the carbonate is seen to occur in the form of a 

 granular aggregate, the grains being of rather uniform size and averaging 

 about 0-08 mm. in diameter. They never show the rhombohedral outlines so 

 common in the dolomites of the Lewis and Galton series. This difference may 

 be easily explained by the fact that all of the Priest River dolomites have been 

 thoroughly recrystallized and now have the structure of true marble, while the 

 younger dolomites seem to have preserved their original sedimentary structure 

 more or less perfectly. The granular dolomite of the thin section is inter- 

 rupted by a few small grains of glass-clear quartz and feldspar. The visible 

 quantity of these impurities matches well the portion of the roek found to be 

 insoluble in hydrochloric acid. About 94 per cent of the rock by weight is 

 made up of the carbonate, which, as shown by the ratio, CaO: MgO (1-46:1), is 

 almost ideal dolomite. 



It happens that a small veinlet of carbonate, cross-cutting the main mass 

 of the rock, appears in the thin section studied. This veinlet is about 1 mm. 

 in diameter. Throughout its visible extent its grains average about 0-02 mm. 



