33 2 



EDWARD STEIDTMANN 



radical differences in the organic development of the early Paleozoic, 

 as compared with the present, and a correlative, fundamental 

 difference in the constitution of the ocean. 



Nor would all students of the pre-Cambrian admit that the 

 profound importance which Daly attaches to the post-Huronian 

 erosion cycle is evident. It was preceded by at least three periods 

 of emergence in the Lake Superior and Lake Huron regions and in 

 Finland. So far as known, any one of them may have contributed 

 as much lime to the sea as the post-Huronian. 



Deposition of magnesian limestones by marine organisms. — 

 Marine organisms deposit magnesium carbonates in small quanti- 

 ties. According to G. Forchhammer, 1 marine shells and corals 

 contain magnesium carbonate in varying amounts from 0.15 to 

 7 . 64 per cent, 1 per cent being rather above the average. In 

 Lithothamnium nodosum, Giimbel 2 found 2 . 66 per cent of magne- 

 sium and 47 . 14 per cent lime, and Hogbom 3 in fourteen analyses 

 of algae belonging to this genus reports from 1.95 to 13.19 per 

 cent of magnesium carbonate. 



The development of magnesium limestones and dolomites by marine 

 leaching. — It has been shown that marine leaching is very effective 

 in concentrating the magnesian content of limestone formations. 

 This is due to the fact that calcium carbonate is several times as 

 soluble as magnesium carbonate, as was first shown by Bischoff. 

 The decrease in calcium carbonate and the corresponding increase 

 in magnesium carbonate resulting from marine leaching is well 

 shown in the table on p. 333, compiled by G. Hogbom. 4 



Hogbom 5 washed the clay marl of Upsula with carbonated 

 water with the following results. The original composition of the 

 marl was calcium carbonate, 18 per cent, magnesium carbonate, 

 1 . 3 per cent. The loss of calcium carbonate was about 50 per 

 cent, while the loss of magnesium carbonate amounted to only a 

 trace. 



In his studies of the marine marls of Sweden, Hogbom found 



1 G. Forchhammer, Neues Jahrb. (1852), 854. 



2 Quoted from Bull. 330, U.S.G.S., 485. 



3 Hogbom, Neues Jahrb. I (1894), 262. 



4 Ibid., 262-74. 5 Ibid., 268. 



