Eocene and Chattahoochee Miocene in Georgia. 447 



I. The almost general presence of a limestone conglomerate 

 at the base of the Chattahoochee, immediately overlying Eocene 

 fossils. This conglomerate is sometimes a breccia, and often 

 like a rock shattered in place ; but more often it consists of 

 clearly rolled pebbles of limestone not distinguishable from 

 the Eocene rock below. 



II. The surface of demarcation between the Eocene and 

 Chattahoochee is very irregular. The Eocene rises island-like 

 into the Miocene. The altitude of the contact differs consid- 

 erably at points but a few miles apart along the strike. Thus 

 the contact is 



180 feet above the sea on Griffin's Creek. 



102 feet above the sea in Glenn's Well, 2-J miles eastward. 



155 feet above the sea in Powell's limesink, s' ill farther east. 



Mr. Eoerste writes me that the fossils at Griffin's Creek 

 correspond with those found in the upper part of the Chatta- 

 hoochee, while those at Glenn's Well, and Powell's limesink 

 correspond with those belonging in the lower half of the 

 Chattahoochee. 



At numerous points in southwestern Georgia there occur 

 between the Eocene and Chattahoochee, masses of columnar 

 corals chalcedonized. They are not present at the majority of 

 the points of observed contact. JSfor has their exact relation 

 to the Eocene or Chattahoochee been observed. Mr. Dall 

 writes me regarding some that 1 sent him, that they belong 

 at the base of the Miocene. The observations lead me to 

 think that they probably grew during the interval between the 

 Yicksburgh and Chattahoochee, upon the submerged Eocene 

 surface. 



Mr. Alexander Agassiz, in " The three Cruises of the Blake," 

 has called attention to the influence of submerged ridges and 

 plateaux upon the growth of organic formations. 



It seems possible that during Miocene time the present 

 plateau of southern Georgia was outlined by submerged 

 islands of the Eocene limestone. The Gulf Stream after the 

 creation of the Central American barrier, found its way back 

 to the Atlantic sweeping over southern Georgia and northern 

 Florida, and supplying the food needed to build up the great 

 organic beds of the Chattahoochee and Chipola. 



As these grew to the surface they formed islands which 

 would explain the presence of the land shells found in the 

 Bainbridge Chipola, and also the beach-worn character of the 

 Chipola fossils at the same locality. And the lower flat-land 

 country of central Georgia may represent the contemporane- 

 ous course of the cold current, carrying less pure water, and 

 less of the nutriment needed to build up organic deposits. 



