NO. 2 PRE-CAMBRIAN ALGONKIAN ALGAL FLORA 89 



cells of Glccocapsa and Gla-otheca, two genera of the Chroococcacese. Sections 

 of the grains reveal the presence of the same forms in the interior of the cal- 

 careous matrix, and it has been concluded on good evidence that the algge 

 are responsible for the deposition of the carbonate of lime of the oolitic 

 grains. By extracting the carbonic acid which they require as a source of 

 food, from the waters of the lake, the solvent power of the water is decreased 

 and carbonate of lime is thrown down. In similar white grains from the 

 Red Sea^ there is a central nucleus in the form of a grain of sand, and cells 

 of Chroococcaceje occur in the surrounding carbonate of lime as in the Salt 

 Lake oolite. 



The analyses of the Michigan Lake deposits show from 2 to 13 

 per cent of magnesian carbonate, the amount varying with the mag- 

 nesian content of the lake water. The amount of calcium and 

 magnesium carbonate is determined by the amount of the two min- 

 erals available for solution in the rocks and soils of the drainage 

 basin tributary to the pond or lake in which the deposits occur. 



Algonkian lakes. — The lakes of Algonkian time were not much if 

 any larger in area than the " Great Lakes " of the St. Lawrence 

 drainage basin and they were much shallower and more laden with 

 mud and mineral matter in solution." 



The area of the Belt terrane in Montana is about 6,000 square 

 miles. This seems large when studying it in the field, but it is only 

 one-fifth of the size of our great fresh-water Lake Superior.' 



DEPOSITION OF LIMESTONE THROUGH THE AGENCY 

 OF ALG^ 



The drainage into the Algonkian lakes undoubtedly afforded all 

 of the soluble mineral matter necessary to account for the lime- 

 stones, siliceou* shales and sodium chloride deposits of the Algon- 

 kian series of formations." 



From a study of the water of the principal rivers of the world 

 Sir John Murray compiled the following table, showing the average 

 amount of mineral matter in solution in one cubic mile of average 

 river water. [Scottish Geol. Mag., Vol. 3, 1887, p. 76.] The propor- 



'Walther, J., Die Korallenriffe der Sinaihalbinsel, Abh. math. phys. C. K. 

 Sachs. Ges., Vol. 14, 1888. 



^ Geology, Chamberlin and Salisbury, Vol. i, 1904, p. 102. 



^ Lake Superior has an estimated area of 32,060 square miles. It is 400 miles 

 long and 160 miles wide. The combined area of the five great lakes is esti- 

 mated at 94,605 square miles. 



* For the areas of the known Algonkian deposits see plate i accompanying 

 report of Van Hise and Leith on pre-Cambrian Geology of North America, 

 Bull. U. S. Geol. Survey, No. 360, 1909. 



