298 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



the Fusispira bed of the Prosser limestone, a southern Minnesota and 

 northern Iowa formation, is locally represented in the Mohawk Valley of 

 New York by a few inches to a few feet of beds at the base of the Trenton 

 limestone. Wherever this thin representative has been observed it is 

 crowded with characteristic Fusispira bed fossils. Partial studies of 

 collections in the National Museum from three localities in the Mohawk 

 Valley have so far brought out over 30 of such species. Similarly, the 

 widely distributed western Femvale Eichmond, whether 1 foot or 25 feet 

 thick, was as a rule immediately charged with its diagnostic fossils. This 

 could not be so if the migration of the faunas had required stratigraph- 

 ically measurable time. 



The rapidity of faunal invasions and migrations is shown perhaps even 

 more convincingly by the early appearance of a new fauna in areas far 

 removed from the point of ingress. The evidence is especially strong 

 when the formation containing the fauna is represented in such distant 

 areas by its fullest known development. For example, in the Stones 

 Eiver at Martinsburg, West Virginia, where this group of limestones 

 attains its maximum known thickness of over 1,200 feet, the fossiliferous 

 layers began either directly above or within a few feet of the unconform- 

 ity at the top of the underlying Beekmantown. And yet this Stones 

 River fauna invaded the continental basins from the Gulf of Mexico. 

 If the migration of the fauna had been slow, as compared to the rate of 

 sedimentation, its appearance at Martinsburg must have been delayed to 

 a time marked by some correspondingly younger bed. 



Many more, and in part varying instances, but all tending to prove 

 that migrations of faunas, unimpeded by physical barriers, progressed too 

 rapidly to be measured in distinguishable geologic time units, might be 

 cited. Lacking space, I must refrain, and trust that the few offered will 

 prove sufficient for present purposes. 



RECURRENCE OF SPECIES AND FAUNAS 



General discussion of recurrence. — The Paleozoic rocks of America 

 afford many instances of species that entered periodically into the faunal 

 history of the interior basins. These species are in perhaps every case 

 also well represented in the rocks of other countries than our own. They 

 were of vigorous stocks and generally very prolific. On account of 

 their vigor they existed, with a minimimi of modification, through a 

 longer period of time than more sensitive species. When conditions 

 were favorable they migrated from their permanent range into the avail- 

 able basins of the interior. In these they spread with great rapidity, as 

 is indicated by their sudden appearance and extreme numerical develop- 



