64 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



Plains, for we doubtless have in the cores of the Belt mountains and Black 

 Hills uplifts, average samples of such rocks as were washed by the sea waves 

 during Altyn time. 



In some thin sections the carbonate is often balled up in spherical or 

 spheroidal bodies, averaging from 0-25 mm. to 0-5 mm. in diameter. These 

 sometimes have an obscure concentric structure, recalling oolite grains. More 

 rarely an imperfect radial arrangement of the minute granules making up 

 each spheroid, is discernible. In all of the observed cases these granules accord 

 in size with tho:e making up the general base of the rock. Certain of the 

 spheroidal bodies recall the ' coccoliths ' such as are precipitated by the action 

 of decomposing albumen on the calcium sulphate of sea water.* There is no 

 evidence that they are foraminiferal tests. 



Often associated with these carbonate concretions are fairly abundant 

 spheroids of cherty matter, averaging about 0-25 mm. in diameter. These may 

 be due to the silicification (replacement) of the carbonate spheroids, or they 

 may also be due to direct chemical precipitation from sea-water. The former 

 interpretation seems the more probable, therewith correlating these microscopic 

 bodies to the manifestly secondary, large nodules of chert found at many 

 horizons in the formation. 



The field and laboratory studies of the Altyn rocks seem, thus, to show 

 that the dolomites and the carbonate base of the subordinate sandy beds are 

 alike the product of chemical precipitation from sea-water. The same may 

 be said of the massive, underlying Waterton dolomite as above described. 



The cause of the precipitation will be more fully discussed in chapter 

 XXIII. on the theory of limestones. For the present it suffices to state, that 

 the cause may possibly be found in the bacterial decay of animal remains on 

 the sea-bottom. The ammonium carbonate generated during such decay reacts 

 on the calcium and magnesium salts dissolved in sea-water, throwing down 

 calcium carbonate and magnesium carbonate. The strong content of carbon- 

 aceous matter in the Waterton dolomite and its occasional occurrence in certain 

 phases of the Altyn formation, may represent the residue of animal carcases. 



Special note should be taken of the nature of the clastic feldspar. It is 

 always remarkably fresh and is mostly a microperthite with typical characters. 

 In view of the unusual nature of this dominant feldspar, it may be used as a 

 sort of fossil in correlating the formation. On stratigraphic grounds it was 

 concluded in the field that the Altyn formation is the equivalent of a part of 

 the Creston formation in the Purcell range and of the Wolf grit and associated 

 members of the Summit series in the Selkirks. The discovery that this special 

 and far from common feldspar is an abundant constituent in all these forma- 

 tions to a degree corroborates this correlation. 



The wonderful freshness of the feldspars suggests that these clastic frag- 

 ments were derived from a terrane undergoing mechanical rather than chemical 

 disintegration. One naturally thinks of an arid climate as supplying the 



* G. Steinmann, Berichte der Naturforschenden Gesellschaft, Freiburg i.B., Band 

 4, 1889. p. 288. 



