62 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Comparison and Conclusions. — Comparing the three analyzed specimens 

 with all the others collected from the Altyn, as well as with the rock-ledges 

 encountered during the different traverses, it appears probable that the average 

 rock of the whole 3,500 feet of beds is composed of about 75 per cent of pure 

 dolomite, about 4 per cent of free calcium carbonate, about 10 per cent of 

 quartz and chert, and about 10 per cent of microperthite, orthoclase (with 

 microcline), plagioclase (only a trace), magnetite, kaolin, and carbonaceous 

 dust, with, possibly, a small proportion of hydrous magnesium silicate. No 

 beds of pure calcium carbonate could be found anywhere in the section, nor 

 any beds of ideally pure dolomite. Even in the most compact specimens 

 examined a notable percentage of clastic quartz and feldspar never failed. 

 Though these rocks are thus impure, like the Waterton formation, they may 

 conveniently be referred to as dolomites. 



One of the most noteworthy facts concerning all these beds, including 

 both the Altyn and Waterton, is the constant size of grain . in the dolomitic 

 base. (Figure 8.) The minute anhedra of carbonate everywhere range from 

 0-005 mm. to 0-03 mm. in diameter, with an average diameter a little under 

 0-02 mm. This is true, no matter what may be the size of the clastic quartz 

 or feldspar. The quartz grains vary from scarcely discernible specks to small 

 pebbles 5-0 mm. or more in diameter. As regards the relative amounts and 

 individual size of these silicious materials, the Altyn formation is quite variable 

 in composition. But its essential base of dolomite is remarkably uniform in 

 grain and in composition. 



This contrast between carbonate base and enclosed clastic materials is 

 worthy of close attention. The quartz grains in different phases of the forma- 

 tion vary from those as small as the average grain of dolomite to those several 

 million times greater in volume, while through all the thousands of feet of 

 strata, the grain of the dolomite itself is rigidly held below an extremely low 

 limit. In most of the slides the quartz and feldspar fragments are thus gigantic, 

 compared to the granular elements of the base and, in most cases, there are 

 very few silicious grains giving the full transition in size between the sand 

 grains and the carbonate grains. Such transitions are to be seen but, as a rule, 

 mo?t of the silicious grains are enormously bigger than the carbonate grains. 

 This steady contrast of size suggests very strongly that the mode of deposition 

 of the quartz and feldspar grains was quite different from that of the enclosing 

 carbonate. The former were unquestionably rolled and rounded by wind-action 

 or under water and were then deposited from water-currents as mechanical 

 precipitates. 



The purity and homogeneity of the carbonate base, its remarkably fine 

 grain, and its perfectly regular microscopic lamination of bedding all point 

 to an origin in chemical precipitation. The sea-water must have been free 

 from mud, the shores furnishing pure sand to the undertow and marine cur- 

 rents of the time. If the carbonate were the result of the mechanical breaking 

 up of shells, coral reefs or older limestones, we should inevitably expect the 

 detrital grains of carbonate to be much larger, or at least much more variable in 



