62 PROCEEDINGS OF THE PHILADELPHIA MEETING 



origin, sucli as mud cracks, cross-bedding, etcetera, is not necessary evidence 

 against tlie continental origin of such deposit. Tliey may be river floodplains 

 or playa deposits. The absence of marine organisms is far more significant. 



Doctor Branson replied : When the deposition of the Red Beds began waters 

 were probably already highly concentrated and unfavorable for life, and the 

 increasing salinity of the waters may have soon rendered the interior seas 

 uninhabitable for most forms of life, and on this account fossils would be few 

 or entirely lacking in the deposits. That such increasing salinity came about 

 is evidenced by the increase of lime in the sandstone from the bottom toward 

 the top, by the limestone deposits at the SOO-foot level, and by the gypsum de- 

 posits near the top. 



Prof. H. E. Gregory took the chair at 2.55 o'ehick. 



NEW POINTS ON THE ORIGIN OF DOLOMITES 

 BY FRANCIS M. VAN TUYL ^ 



(Ahstract) 



A careful study of the dolomites of the upper Mississippi Valley was under- 

 taken for the Iowa Geological Survey during the field season of 1912. More 

 recently a grant from the Esther Herrmun Research Fund of the New York 

 Academy of Sciences has made possible much more extensive studies of the 

 dolomitic limestones of the Eastern and Central States. The present prelimi- 

 nary paper is intended merely to set forth some of the more important results 

 of these studies. 



Existing theories of the origin of dolomite were briefly considered, after which 

 the problem was attacked from three standpoints, namely, the experimental 

 evidence, the field evidence, and the petrogra])hic evidence. The conclusion 

 was reached that the great majority of the dolomites, ranging in age from the 

 Cambrian to the present, have resulted from the replacement of limestones 

 before they emerged from the sea. The replacement need not be accompanied 

 by shrinkage, as formerly supposed, but may proceed according to the law of 

 equal volumes, as enunciated by Lindgren. Furthei-more, certain cases of ap- 

 parent inter-stratification of limestone and dolomite cited as evidence in favor 

 of some primary theory are rather pseudo-inter-stratifications, which have 

 resulted from selective dolomitization. Examples of limestones mottled with 

 dolomite were interpreted as representing an incipient stage in the process of 

 alteration. Organic factors have exerted a selective influence in some cases 

 of mottling, but in others the phenomenon is jturely inorganic. 



Eead in full from manuscript. 



1 Introduced by Stuart Weller. 



