REPORT OF THE CHIEF ASTRONOMER 



12? 



SESSIONAL PAPER No. 25a 



on the Commission trail, about one thousand yards west of the main fork of 

 the river, has been chemically analyzed. Its description will serve to show 

 the general nature of the typical calcareous part of the formation. 



On the fresh fracture the rock is light gray, compact, and thin-bedded, though 

 platy because of the cementation of many laminae of varying composition. 

 The weathered surface is generally of a still paler gray colour, but for a depth 

 of one or two millimetres below the surface there is usually a shell of altered 

 rock of a brown or buff colour. The decolourization at the surface is doubtless 

 an effect of leaching by vegetable acids. 



Under the microscope the rock is seen to be composed of carbonates, 

 quartz, feldspar, sericitic mica, a little green biotite, and small grains of 

 limonitized iron ore. These constituents are named in the order of decreasing 

 abundance. The carbonate grains vary from 0-01 mm. or less to 0-03 mm. in 

 diameter and average about 0-02 mm. They never appear to have rhom- 

 bohedral development. The quartz and feldspar grains which are, doubtless, 

 in largest part of clastic origin, vary from 0-02 mm. to 0-1 mm. or more in 

 diameter, averaging about 0-06 mm. The dominant mica, sericite, is not dis- 

 tributed uniformly but is most abundant in rather sharply defined laminae of 

 specially fine grain. Such laminae were evidently more purely argillaceous 

 than the remainder of the rock. No true argillaceous material can be dis- 

 cerned in thin section; the sediment has been very largely recrystallized and. 

 its insoluble base is a metargillite. 



The percentages in Professor Dittrich's chemical analysis (specimen No. 

 1179) are not very different from those roughly deduced from microscopic 

 study : — ■ 



Analysis of type specimen, Creston formation, Eastern Phase. 



SiO 



A1 2 3 



FeX> 3 - 



FeO 



MgO 



CaO 



Na,0 



K 2 



H 2 at 110°C. . 

 H 2 above 110°C. 

 C0 2 



Mol. 



51-65 



•861 



7-85 



077 



1-74 



•Oil 



•98 



•014 



3-67 



■092 



15-02 



•268 



2-69 



•044 



1-38 



•015 



•09 





1-81 



•100 



13-05 



•297 



Sp. gr. 



99-93 

 2-654 



Insoluble in hydrochloric acid. 

 Soluble in 'hvdrochloric acid: 



Fe 2 3 



A1 2 3 



CaO 



MgO 



66-21% 



1-92 

 2-02 



12-88 

 2-41 



Assigning the soluble lime and magnesia to the carbonates, the remainder 

 of the lime to the anorthite molecule, the soda to the albite molecule, the 



