690 



SCIENCE 



[N. S. Vol. XLIV. No. 1141 



dolomite and limestone; the presence of im- 

 perfectly dolomitized oolite beds in dolomites; 

 the occurrence of mottled limestones grading 

 gradually into dolomite, and many other fea- 

 tures can only be accounted for by assuming 

 that dolomitization took place after all of the 

 beds involved were deposited, or at least in the 

 closing stages of their deposition. When, how- 

 ever, a pure limestone member succeeds a dolo- 

 mite member, known to be an alteration prod- 

 uct, conformably, the contact line being reg- 

 ular and continuous over wide areas, it can 

 not be assumed that this relationship has re- 

 sulted from the alteration of the lower bed 

 after both beds were deposited. The " Lower 

 Buff beds " of northeastern Iowa, which con- 

 sist of dolomite with occasional minute lime- 

 stone remnants, are abruptly followed by the 

 pure limestone of the " Lower Blue beds " over 

 hundreds of square miles, the transition from 

 one into the other taking place through only 

 a few inches of imperfectly dolomitized lime- 

 stone. 



Moreover, the tendency of some limestones 

 to be more highly dolomitic in their lower por- 

 tions and to become progressively less dolo- 

 mitic upwards, must also be regarded as lend- 

 ing support to the theory of progressive dolo- 

 mitization. Orton and Peppel 6 state that the 

 Delaware and Columbus limestones of Ohio 

 are more dolomitic in their lower than in their 

 upper portions. 



But even if it should be positively shown 

 that dolomitization can go on at low concen- 

 trations, all must agree that it would proceed 

 not only much more rapidly, but also more 

 completely at higher concentrations. With 

 reference to the question whether the ancient 

 seas which accomplished such extensive dolo- 

 mitization were more concentrated than the 

 modern ones or not, little can yet be said. On 

 this point we must rely solely upon inference. 

 Steidtmann 7 has presented evidence to show 

 that the ancient seas were more highly mag- 

 nesian than those of to-day. From independ- 

 ent lines of reasoning based upon paleogeo- 

 graphic evidence the writer is also led to be- 



« Ohio Geol. Survey, 4th ser., Bull. 4, p. 165. 

 i Jour. Geol., Vol. 19, pp. 323 and 392. 1911. 



lieve that the magnesia content of the ancient 

 seas may have been at least temporarily 

 greater than at present. Let us consider the 

 conditions obtaining in a constricted interior 

 sea from which limestone is being deposited 

 on a great scale. Fresh quantities of lime and 

 magnesia and other salts are being introduced 

 into this interior sea both by influx from the 

 open ocean and from the streams draining the 

 land. Now lime is constantly being depleted 

 from this inland sea by lime-secreting organ- 

 isms, while the magnesia and other salts tend 

 to accumulate. It seems possible, then, that 

 during a long period of limestone formation 

 under these conditions magnesia might ac- 

 cumulate in considerable excess and that ere 

 long extensive dolomitization might set in and 

 continue until equilibrium was once more es- 

 tablished. 



Applying this theory now to the strati- 

 graphic col umn, we actually find that many 

 periods of extensive limestone formation in 

 interior seas may be correlated with periods 

 of extensive dolomitization. Witness the 

 great dolomite masses of the Cambrian of the 

 Appalachian province, and of the early Ordo- 

 vician and of the Niagara. 



As further evidence that the early seas 

 which accomplished extensive dolomitization 

 may have been temporarily concentrated, at- 

 tention may be called to the fact that these 

 seas in many instances were retreating and 

 contracting towards the last, and that unless 

 they were freely connected with the open 

 ocean, evaporation under arid or semi-arid 

 conditions might give rise to a considerable 

 increase in salinity. Such a condition would 

 seem to apply especially well to the Niagaran 

 sea. Paleogeographic studies have shown that 

 this sea became very much contracted towards 

 the close of this epoch, and Clarke and Euede- 

 mann 8 have concluded that the Guelph fauna 

 must have inhabited a sea of abnormally high 

 salinity. The latter fact considered in con- 

 nection with the evidence of widespread dolo- 

 mitization in the later stages of the Niagara 

 seems significant. Francis M. Van Tuyl 



Geological Laboratory, 

 University of Illinois 



s Mem. N. Y. State Museum, No. 5, p. 117. 



