PRINCIPLES OF PALEONTOLOGIC CORRELATION 683 



was a time of encroachment of the land on the sea, and only 

 occasionally, when the sea had temporarily reclaimed its own, 

 are marine faunas found in this formation. But when these 

 occur, they are often extra-provincial, and occasionally extra- 

 regional in origin, and thus give a secure basis for correlation 

 with those regions where marine conditions still prevailed. Such 

 a state of affairs existed in western North America and in east- 

 ern Europe during the time of the Coal Measures ; in these 

 regions the sea transgressed over the land areas, and allowed 

 the marine faunas to become widely distributed. By intermit- 

 tent subsidence of the low-lying coal swamps an intercalation of 

 marine with freshwater deposits took place, allowing accurate 

 correlation between the two facies. 1 And occasionally these 

 oscillations have been something more than local events, for they 

 have brought in exotic faunas, as in the case of the belated 

 immigration of Pronorites cyclolobits and Co?iocardium aliforme in 

 the Lower Coal Measures of America, or the appearance of Gas- 

 trioceras and Paralegoceras in the European waters long after they 

 had appeared in America. The greatest of these disturbances 

 was the Appalachian revolution, which at the beginning of 

 Permian time raised finally above water the continental borders 

 of the old Appalachian land mass, and left only a comparatively 

 small basin for the Permian sea. This rising of the Mississippi 

 valley region was undoubtedly accompanied by sinking else- 

 where, for a very similar exotic fauna appeared simultaneously 

 in the American, the European, and the Asiatic region, and 

 mingled with the preexisting local faunas, giving one of the 

 most distinctive paleontologic zones yet known, 



During the Lower Trias the Arctic, the American, and the 

 Oriental regions had closely allied faunas, and might be grouped 

 together in contrast with the Mediterranean. At this time of 

 transgression and readjustment of geographic boundaries we 

 have the widely distributed fauna of the Meekoceras zone, dis- 

 tinctly recognizable in India, Siberia, and western America. 



'J.P.Smith: Jour. Geol., Vol. II, No. 6, 1894, The Metamorphic Series of 

 Shasta County, California. 



