76 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



This analysis cannot be calculated quite as readily as those of the Altyn 

 dolomites; the alkalies are here not assignable with certainty to definite feld- 

 spars. For the purpose of comparison, however, the same method of calculation 

 has been applied here, giving a ' norm ' wherein the soda is assigned to the albite 

 molecule and the potash to the orthoclase molecule, just as the alkalies are 

 assigned in calculating the ' norms ' of igneous rocks. The carbonates have been 

 calculated directly from the analysis of the soluble portion. The results are given 

 in the following table, which shows the ' mode ' for the rock as far as the car- 

 bonates are concerned, the other constituents being more arbitrarily treated : 



Calcium carbonate 35-3 



Magnesium carbonate 18-8 



Silica 28-0 



Orthoclase molecule 7-2 



Albite molecule 4-2 



Magnetite. 2-6 



Remainder 3-9 



100-0 



The proportions of the carbonates correspond to 41-2 per cent of normal 

 dolomite and 12-9 per cent of free calcium carbonate. 



This excess of calcium carbonate is probably not due to its having been 

 introduced into the molar-tooth rock from other beds. The magnesian portions 

 of the molar-tooth rock effervesce somewhat with cold, dilute acid; it seems 

 simplest to believe that the calcium carbonate was there originally in excess 

 and dates from the time of the deposition of the sediment. The two carbonates 

 together are seen to make up about 54 per cent of the rock. 



The high percentage of water (above 110°C) is of interest as showing that 

 this metamorphic agent, even at the present time, is enclosed in sufficient 

 amount to explain the solutional effects illustrated in the molar-tooth structure. 

 The content of carbon, low as it is, is partly responsible for the normally dark 

 tint of the fresh rock. 



Different as the Siyeh dolomite limestone and the Altyn dolomite are in 

 field-habit, the two types are yet similar in several important respects. In each 

 the carbonate base has a remarkably fine and homogeneous grain, with no 

 suggestion in either case that the carbonate is of clastic origin. In each case 

 the dolomitic grains tend to assume the rhombohedral form. Their average 

 diameter is sensibly identical in size with that of the average calcite grains 

 composing the lenses, pencils, and stringers of the molar-tooth rock. This 

 average diameter is also practically equivalent to that characterizing the gran- 

 ules which compose each of the egg-like bodies forming occasional thin beds 

 of oolite in the Siyeh and neighbouring formations. There is no doubt of the 

 chemical origin of the latter, nor can there be doubt that the calcitic partings 

 of the molar-tooth limestone were gradually crystallized out from water solu- 

 tions. It would seem next to incredible that these three associated rocks, 

 characterized by the same average size of constituent carbonate particles, could 

 have in two cases an origin in chemical precipitation and. in the third, an 



