POTSDAM PERIOD. 201 



(2.) Limestones. — The limestones of the Silurian and later ages 

 have nearly all been made through the wear and accumulation of 

 shells, crinoids, and corals, or the calcareous relics of whatever life 

 occupied the seas. The great limestone formations of existing coral 

 seas are modern examples of the process. 



Among the beds of the Potsdam period, the magnesian limestone 

 strata of the Quebec group contain numerous fossils, and thus show 

 that they are marine, and that they have the origin above men- 

 tioned. The extensive magnesian limestones of the Mississippi 

 valley have the same composition, and are similar in compactness : 

 and the natural inference is that they were also of organic origin. 

 But over extensive regions they do not contain a single fossil. Yet 

 it is to be remembered that the sea which grinds pebbles and 

 sand and makes fine sandstones may also grind shells and make an 

 impalpable limestone. This is abundantly exemplified in coral- 

 regions ; for a large part of the limestone there made of corals and 

 shells is as compact and unfossiliferous as the magnesian limestone 

 in question. 



The only other mode of origin is by chemical deposition. This 

 could not have taken place in the open seas ; for, owing to the 

 oceanic currents, the waters have a remarkable uniformity of com- 

 position, and no- local depositions can take place. It requires, there- 

 fore, an elevation above the sea, and the existence of calcareous 

 mineral springs, — and springs on a wonderfully vast scale, for a 

 formation as extensive as the one in question. Such a condition 

 of things is improbable. Moreover, the depositions would have a 

 structure wholly unlike that of the magnesian limestone. Who- 

 ever has seen the travertine beds of Tivoli — which are the largest 

 of the chemical calcareous deposits formed in the present era — will 

 appreciate the wide distinction between a mass made up of a series 

 of incrustations curving with all sorts of fantastic irregularities, 

 and the dense, even-grained limestone of the Calciferous epoch. 

 The oolitic structure of part of this limestone has a parallel in the 

 oolitic coral rock of Key West, which is also without imbedded 

 corals or shells. 



It is not impossible that the strata may have been made of 

 microscopic organisms : the shells of Ehizopods, which have con- 

 tributed so largely to chalk (though not commonly distinguishable 

 in the mass), have been detected in the Lower Silurian of Russia, 

 in "green-sand" like that of the Potsdam period (pages 174 and 

 176), and abundantly in the "green-sand" of the Cretaceous forma- 

 tion ; so that the existence of this material leads the geologist to 

 suspect at once that of the Rhizopods. 



