620 E. BLACKWELDER ORIGIN OF BIGHORN DOLOMITE OF WYOMING 



must have been largely curtailed, and since the ocean even today carries 

 but a small working balance of lime, constantly being drawn on by organ- 

 isms, it is not entirely improbable that in such expanded epicontinental 

 seas organisms were unable to obtain sufficient lime carbonate for their 

 needs. Daly 18 argues that in the early eras lime must have been practi- 

 cally absent from the sea-water, because of the action of alkali carbonates 

 in removing it from solution; but he concludes that on this account the 

 organisms were unable to provide themselves with calcareous hard parts — 

 on the assumption that they could use no other salt. It is a fact long 

 known to agricultural chemists that most land plants require potassium 

 salts in their metabolism and can not use sodium compounds as substi- 

 tutes therefor. There is good reason 19 to believe that marine plants, on 

 the contrary, demand sodium. It has been suggested that this fact merely 

 reflects a necessary adaptation to environment, for potassium is the alkali 

 that remains in largest quantity in the soils which accumulate on land, 

 and sodium is by far the most abundant alkali in the sea. Following out 

 this line of thought, it would not seem strange if marine organisms were 

 able to make shift with dolomite as skeletal material when the supply of 

 calcium was inadequate. Nevertheless, the invariable deficiency of mag- 

 nesia in existing shells leads me to regard this third hypothesis with very 

 little favor. 



Eeserving the fourth suggestion for more extended examination, we 

 may next consider whether the Bighorn strata were deposited as limestone 

 and afterward converted into dolomite by the action of percolating under- 

 ground waters, of which the pervasive effects on rocks in general are well 

 understood. There are apparently two methods by which this change 

 could be accomplished: (a) by leaching out lime and (b) by replacing 

 lime with magnesia. 



If this change had been brought about solely by the leaching of lime 

 carbonate from a slightly magnesian limestone, thus increasing the rela- 

 tive proportion of magnesia, we should now find caverns, either empty or 

 refilled, disturbed beds due to slumping, and other evidences of great re- 

 duction in volume. All these are absent. If it involved only metasoma- 

 tism (the substitution of magnesia for lime, ion by ion), the compactness 

 of the rock is explicable, for the dolomitization of calcite in this way 

 should cause a reduction in volume of the mineral grains or, in other 

 words, pore space of only about 4.7 per cent. A careful determination of 

 the porosity ©f a typical specimen of the Bighorn dolomite has been made 



18 R. A. Daly : Limeless ocean of pre-Cambrian time. Am. Jour. Sci., 4th ser., vol. 23, 

 1907, p. 109. 



19 W. J. V. Osterhout : Plants which require sodium. Botanical Gazette, vol. llv, 1912, 

 pp. 532-536. 



