110 H. A. Daly — Limeless Ocean of Pre-Cambrian Time. 



dolomite is a fact hardly to be reconciled with these meta- 

 morphic theories. The metamorphism is. by these theories, 

 accomplished through the activities of circulating under- 

 ground waters ; yet it seems impossible that such wholesale 

 metamorphism could leave the original bedding so well 

 marked. The alternation of clean-cut beds as described is a 

 prominent fact illustrated, for example, in the pre-Cambrian 

 formations of Montana and British Columbia. The facts of 

 the field speak rather for an original deposition of the two 

 carbonates arranged in very nearly their present relations. 



It is scarcely necessary to dwell on the effect of burial on 

 the chemical precipitate of basic magnesium carbonate. Pres- 

 sure and a heightened temperature have gradually driven out 

 the water of crystallization. The simultaneous formation of 

 the double carbonate, dolomite, might be expected where both 

 carbonates are present in large amount. The shrinkage con- 

 sequent on the loss of water of crystallization amply accounts 

 for the cavernous structure often seen in dolomites. 



In conclusion, it appears that the hypothesis here proposed 

 bears its third principal test so far as the carbonates of calcium 

 and magnesium are concerned. It involves the precipitation 

 of both carbonates from sea- water through the decay of animal 

 matter. The magnesium carbonate should have been most 

 abundantly thrown down in pre-Cambrian time ; its precipita- 

 tion must have been lessened through Paleozoic and Mesozoic 

 time, and has reached its minimum since the abysses of the 

 ocean became abundantly tenanted with scavengers. The 

 study of 700 limestone analyses, selected by the writer to repre- 

 sent the different periods, seems to show, in fact, that the 

 ratio, MgO : CaO, in the carbonate rocks is very high in pre- 

 Cambrian terranes, lower in Paleozoic and earlier Mesozoic 

 series, and much lower still in later deposits.* 



Origin of the Lake Superior iron ores. — The many 

 thorough studies that have been made of the several iron-pro- 

 ducing districts about Lake Superior have demonstrated two 

 original sources for the ore. One of these is the iron-silicate 

 " greenalite," which has many points of resemblance to the 

 modern glauconite and may be of chemical origin. The other 

 source of the valuable oxides is iron carbonate, which still 

 occurs in the form of distinct beds or in the form of an essen- 

 tial constituent of chert-bands or of frankly detrital deposits. 

 It is worthy of consideration whether the siderite may be, in 

 part at least, due to precipitation by ammonium carbonate of 

 animal origin. If, for example, we imagine either chalybeate 

 springs or a river furnishing special amounts of ferrous sul- 



*Cf. C. E. Van Hise, Treatise on Metaniorphisrn, p. 801, 1904; Chamber- 

 lin and Salisbury, Geology 1904, vol. i, pp. 360, 404. 



