FAUNA OP ORTHAULAX PUGNAX ZONE. 



the Chattahoochee River a stratum visible, but not there fossilifer- 

 ous, is continuously traceable to Rock Bluff, where it contains charac- 

 teristic Oak Grove species, so that the position of the latter in the 

 Tertiary column is definitely fixed. 



About 24 per cent of the Oak Grove fauna is identical' with that 

 of the Chipola marl, but characteristic species like Orthaulax have 

 vanished. The sweeping nature of the change caused by the Mio- 

 cene invasion of cold water is shown by the fact that of the Oak 

 Grove species less than 1 per cent survive in the fauna of the superin- 

 cumbent Miocene beds. 



The premonition of Miocene conditions is shown, however, in the 

 Oak Grove fauna by the appearance of a large Lyropecten and some 

 few other analogous species. 



After the Miocene a recurrence of warmer conditions brought back 

 in the Pliocene of Florida a good many of the species which had 

 been exiled by the inflow of cold Miocene waters. 



The above summary of our knowledge at the time of publication in 

 1903 indicates the relations of the Orthaulax bed to adjacent Ter- 

 tiary faunas as understood at that period. 



The next important attempt to classify the Florida Tertiary beds 

 which are associated with the Orthaulax bed is found in the Report 

 on the Geology of Florida with special reference to the Stratigraphy, 

 by George C. Matson and F. C. Clapp.^ In this report the attempt 

 is made to consider the peninsular part of Florida as an inherent part 

 of the coastal plain and to explain its geological histor}'' as dependent 

 on the orogeny of the continental region. In the work of the present 

 writer the present peninsula of Florida is regarded as independent of 

 the Eocene continental border, to which it became attached only after 

 the close of the Miocene, and as related to a group of late Eocene or 

 Oligocene islands separated by a wide strait both from the continent 

 and from Cuba and having its own genetic history, which in Tertiary 

 time only in the very widest and least effective sense depended on the 

 continental movements. 



The present writer has shown by railway levels that the peninsular 

 part of Florida is marked by two principal northerly and southerly 

 low ridges with a shallow basin between them; a fact obvious from 

 the distribution of rivers and lakes on any detailed map ; and by the 

 location of the fossiliferous strata, that the whole peninsula has a 

 gentle tilt from east to west, thereby causing the encircling deposits 

 about the original islets to dip under the Gulf of Mexico on the west- 

 ern shore of the peninsula. Consequently he feels unable to accept 

 without some evidence the hypothesis that the central basin is an 

 eroded anticlinal arch. Such evidence has not been made public. 



While exploring in Florida I learned that wells sunk in the west- 

 ern ridge reached rock only at a depth corresponding roughly with 



1 Florida State Geol. Survey, 2nd Ann. Rept., 1908-9. pp. 25-161, Jan., 1910. 



