650 



DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



buried beneath the waters of the Atlantic. On the east and west of the 

 continental area the pre-Cambrian land formed the mountain region; and 

 over the interior a plateau existed that at the beginning of, or a little before, 

 Upper Cambrian time was much as it is to-day. Subsequent mountain- 

 building has added to the bordering mountain ranges, but I doubt if the 

 present ranges are as great as those of pre-Cambrian time that are now known 

 only by more or less of their truncated bases. The Interior Continental 

 area was outlined then and it has not changed materially since. Its 

 foundations were built in Algonkian time on the Archean basement, and 

 an immense period of continent growth and erosion elapsed before the first 

 sand of Cambrian time was settled in its bed above them.'f 



Following the last world-wide, orogenic paroxysm of pre-Paleozoic time, 

 there was a long interval of more or less perfect baselevelling. In the process 

 thousands of cubic miles of rock were weathered and a large proportion of their 

 mass went, in solution, to the sea. At least three conditions were present to 

 favour a special enrichment of the sea water in soluble salts of calcium. Great 

 volumes of basic volcanic rocks were now for the first time exposed to weather- 

 ing, with the necessary evolution of lime salts; the thick limestones chemically 

 precipitated in Azoic ( ?) and earlier Eozoic periods were, now for the first time, 

 exposed to solution in rain water; and the areas of the lands and of drainage 

 basins, with all their assemblage of weathering heterogeneous rocks, were probably 

 greatly increased over their magnitudes in former times. 



It is possible to obtain some idea of the quantitative influence of the first 

 two conditions. Hanamann's careful investigation of the Bohemian rivers has 

 clearly shown the important relation between the lithological nature of a terrane 

 and the content of calcium in the waters issuing from the terrane.:}: His results 

 are summarized in the following table in which the figures for each terrane 

 represent averages of several analyses :- — 



Table XXXVITI. Calcium and magnesium in Bohemian rivers 



Waters from. 



Granite 



Phyllite 



Mica schist 



Basalt 



Cretaceous (largely limestone) 



Calcium 



in parts 



per 



million. 





7 



73 





5 



72 





9 



33 





68 



84 





38 



38 



Magnesium in 

 parts per 

 million. 



33 

 •41 

 •76 

 •76 

 •36 



Ratio of calcium to 



magnesium. 



3 



32:1 



2 



37:1 



2 



48:1 



3 



49:1 



4 



25:1 



t C. D. Walcott, 12th Ann. Rep. U.S. Geol. Survey, 1891, p. 562. 



XRef. in F. W. Clarke, Data of Geochemistry, Bull. 330, U.S. Geol. Survey, 1908, 

 P. 79 ; Cf . also A. L. Ewing-, Amer. Jour. Science, Ser. iii, Vol. 29, 1885, p. 29. 



