REPORT OF THE CHIEF ASTRONOMER 



173 



SESSIONAL PAPER No. 25a 



Weighting each, value according to the thickness of the corresponding- 

 formation, the average specific gravity of each of the four great series has 

 been calculated. The final result affords a fair estimate of the actual density 

 of the Rocky Mountain, Purcell, and Selkirk ranges where these mountains, 

 as at the Forty-ninth Parallel, are almost entirely composed of the pre-Silurian 

 geosynclinal rocks. 



On account of the thorough induration and compactness of nearly all the 

 specimens, the usual trouble arising from included air has not been seriously 

 felt. Other sources of observational error were partly obviated by the use of 

 selected, whole hand-specimens (200 to 1,000 grams in weight), whereby rather 

 reliable averages for the formations were secured. 



The following tables embody the principal results : — 



Table V. — Showing the calculated average densities of the four series, including 



the interhedded lavas. 





Number of spe- 

 cimens 

 measured. 



Thickness of 



series in 



feet. 



Average 



sp. gr. of 



series. 







39 

 73 

 35 

 55 



13,720 

 12,460 

 20,300 

 32,050 



2737 



Purcell series, western 



phase : 







2-711 

 2-701 " 

 2750 











202 





Table VI. — Showing the calculated average densities of the four series, excluding 



the interhedded lavas. 





Number of spe- 

 cimens 

 measured. 



Thickness of 



sediments 



in feet. 



Average 





S P-. S T - 

 of sediments. 





37 

 67 

 35 

 40 



179 



13,400 

 12,150 

 20,300 

 26,050 



2-734 





2-708 

 2701 

 2-710 







Table VII. — Showing the calculated average densities of that part of 

 geosynclinal which corresponds to the full thickness of 

 sediments in the Galton series. 



Average sp. gr. 



Lewis series 2-726 



Galton series 2-708 



Purcell series ' 2-702 



Summit series 2-705 



the 



