REPORT OF THE CHIEF ASTRONOMER 63 



SESSIONAL PAPER No. 25a 



size than the actual particles. There must have been changes of sea level or 

 of depth of water, or changes in both during the accumulation of these 3,500 

 feet of sediment. It is virtually inconceivable that, throughout such changes, 

 the size of carbonate particles broken off from either shells or bed-rock and 

 brought hither by currents, should always average from 0-01 to 0-02 mm. in 



Fjgure 8. — Diagrammatic drawing from thin section of typical sandy 

 dolomite of the Altyn formation. Round (wind-blown ? ) grains of 

 quartz (clear white) and much less abundant microperthitic feldspar 

 (transverse lines) drawn to scale. The dots represent, on the same 

 scale, the size of the extremely minute carbonate granules com- 

 posing the matrix of the rock. Diameter of circle 4'5 mm. 



diameter and never reach diameters above 0-05 mm. or thereabouts. If the 

 carbonate base were of detrital origin one should expect to find variations in 

 its grain as he approached or receded from the source of detrital supply. Such 

 variation is not to be found at any of the sections yet studied in the Altyn 

 formation. So far as the pre- Altyn rocks are known there seems to have been 

 in the adjacent Cordilleran region no magnesian limestones of anything like 

 the volume required to furnish, from their mechanical disintegration, the 

 material for the thousands of cubic miles of carbonate represented in the Altyn. 

 The same may be said of the pre-Altyn formations underlying the Great 



