196 HALL AND SARDESON — THE MAGNESIAN SERIES. 



result can now be only conjectured. In forming that conjecture geolo- 

 gists may note the beds of the Magnesian Series as they exist today after 

 having remained raised above the sea for a time much longer than that 

 involved in the accumulation and covering with Upper Cretaceous and 

 successive Tertiary beds of the Middle Cretaceous limestones of Texas.* 



Removal of Calcium Carbonate and its Effects. — Thus we see that to b^ing 

 the Magnesian Series into their present chemical condition an enormous 

 reduction in the quantity of calcium carbonate, and consequently in the 

 bulk of the formations, must be assumed. No figures are possible which 

 will more than approximate to accuracy. 



Elie de Beaumont calculated that the removal of every other molecule 

 of CaCOg and its replacement by a molecule of MgCOg would reduce the 

 volume of the mass to the extent of 12.1 per cent.f Thus every 100 feet 

 of the present thickness of the Oneota formation would represent an 

 original thickness approaching 112 feet, since it has not yet become a 

 typical dolomite. 



While de Beaumont's calculation might prove a very reasonable one 

 for certain localities, the conditions which obtain in the northwestern 

 states do not appear to conform to such a theory. Even if the dolomites 

 were formed in the deep sea, as some investigators assume, the long and 

 constant separation of the calcium carbonate, continuous since the eleva- 

 tion of the rocks above the sealevel, effected through the agency of per- 

 colating waters and now going on, would have reduced the beds to a 

 very great extent from tlieir original or acquired chemical and physical 

 conditions. But de Beaumont's theory of molecular replacement is not 

 satisfying. It is difficult to understand from whence came the enormous 

 quantities of magnesium carbonate which was necessary for such conti- 

 nent-wide replacement as must have taken place if all the Paleozoric 

 dolomites known to geologists were formed in that way. It is equally 

 difficult to understand how the replacement on such a broad plan could 

 have been carried on beneath the sea at great depths. 



Geikie also very strongly hints that the cavernous condition shown by 

 many dolomites is not sufficiently accounted for by this calculation. 



The porous condition of the dolomites under consideration has already 

 been pointed out. This character was considered by Professor Dodge in 

 his examination of the physical characters of the Minnesota building 

 stones. J It is found that the average increase of weight by a four days' 

 saturation in water in the case of ten samples of dolomites and dolomitic 



*E.T. Durable: Geology of the Valley of the Middle Rio Grande, Bull. Geol. Soe. Am., vol. 3, 

 p. 230. Durable makes the Austin beds the lowest liraestone of the Upper Cretaceous of the Colo- 

 rado section. 



tGeiliie : Text-booli of Geology, third edition, 1893, p. 321. 



J Geol. and Nat. Hist. Survey of Minn., Final Rep., vol. i, pp. 195-203. 



