670 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



DIRECT EVIDENCE OF THE CHEMICAL PRECIPITATION OF THE CARBONATE ROCKS IN THE 

 PRIEST RIVER AND BELT-CAMBRIAN TERRANES. 



Finally, we may turn to more direct evidences that the vast pre-Silurian 

 limestone and dolomite deposits encountered in the Boundary belt, were origin- 

 ally chemical precipitates on the sea floor. This conclusion has been stated at 

 several points in the detailed description of the Waterton, Altyn, Siyeh, Shep- 

 pard. and Creston (Eastern phase) formations, and it is only necessary to sum- 

 marize the facts. 



One of the leading arguments is the argument by exclusion. Some fourteen 

 thin sections of typical phases of these formations have been specially studied 

 under the microscope. The specimens were taken at localities ranging from 

 Waterton lake to the Yahk river, a distance of 120 miles, and at horizons 

 ranging through 12,000 feet of the Lewis series and through about as great a 

 thickness in the Purcell series. In spite of such highly varied positions in the 

 sedimentary prism, the grain of the carbonate rocks, as shown in the thin 

 section and as implied in the never-varying compactness of the rock in the 

 field, is most extraordinarily uniform. 



The constituent grains are either idiornorphic and rhombohedral or anhedral 

 and faintly interlocking. The former are everywhere of nearly constant average 

 diameter, ranging from 0-01 to 0-03 mm., with an average of 0-02 mm. The 

 anhedral grains range from 0-005 mm. or less to 0-03 mm., averaging about 

 0-015 mm. 



This fineness and uniformity of grain persists not only in the compact 

 Siyeh and Sheppard beds but also throughout the many beds of the Altyn, where 

 coarse quartz and feldspar sands and pebbles are abundantly distributed in the 

 carbonate base. Neither horizon nor distance from the old shore-line sensibly 

 affects the singularly monotonous grain. In view of these facts regarding the 

 the grain, in view of the rhombohedral forms of the one class of granules, and 

 in view of the fact that there is no known pre-Belt carbonate formation at all 

 adequate to furnish the materials for these tens of thousands of cubic miles of 

 magnesian limestones, it seems impossible to credit them with a clastic origin. 

 On the other hand, all the above-mentioned facts and the character of the 

 bedding, which is often paper-thin and clean-cut as befits a precipitate, point 

 directly to a chemical origin. 



The argument is further strengthened by the fact that the much older Priest 

 River dolomites seem to have bad nearly the same grain and other general 

 characteristics of the limestones in the overlying prism. 



Secondly, it is important to note that the average diameters of the carbonate 

 granules are of the same order as the average diameters of calcite and dolomite 

 crystals which are unquestionably due to chemical precipitation from sea water 

 or saline solutions at ordinary temperatures. Cullis has shown that the calcite 

 granules deposited from sea water in the cavities of the Funafuti corals have 

 average diameters of from 0-02 mm. to 0-03 mm.; also that the dolomite crystals 

 which have gradually replaced the aragonite and calcite of the coral deposits, 



