SECOND ANNUAL REPORT—STRATIGRAPHIC GEOLOGY. 
77 
surface configuration is often determined, in part, by the character 
of the superficial sands and clays of post-Oligocene age. The lime¬ 
stones of the Chattahoochee formation are less soluble than those of 
the underlying Vicksburg group; and, hence, they contain fewer un¬ 
derground streams, which give a characteristic sink-hole topography. 
However, underground streams, sink-holes and natural bridges are 
by no means rare. Where the limestones belonging to the Chatta¬ 
hoochee formation are thin the topography is often the combined re¬ 
sult of solution of the Lower Oligocene limestones and the protection 
of the ridges and hills by the more durable Upper Oligocene limestones. 
Paleontologic Characters:—At some localities in southern Georgia 
the basal layers of the Chattahoochee formation supply many corals 1 
and the lower part of this formation contains Orthaulax pugnax, a 
gastropod which is characteristic of the “silex bed” at Tampa. This 
locality has been studied by Vaughan , 2 whose description emphasizes 
the existence of an erosion interval between the deposition of the 
Vicksburg and Apalachicola groups, and shows the existence of a 
fossil coral reef. Of this coral reef, Vaughan says: 
My estimate is that there are between twenty-five and thirty species. 
This is the richest fossil coral fauna known from any one locality of the 
Continental North American Tertiaries. However, the state of preservation of 
the specimens is not always satisfactory, and it may not be possible specifically to 
describe all of them. 
The particular interest of this fauna does not lie in its richness, but in its 
geologic import. The Tertiary coral faunas of the United States below the 
Chipola horizon were very isolated, no species from the continent, excepting the 
Orbicella mentioned, being found in any other area. This fauna is distinctly 
Antiguan in types. Besides the Orbicella referred to, there is a very large-celled 
Orbicella, very close to O. crassilamellata (Duncan), if not identical with that 
species, found abundantly at Russell Spring. An Astrocoenia is extremely close 
to A. ornata of Duncan from Antigua. The same remark will apply to the 
Stylophora and Alveopora. 
From this field examination it appears that the reef corals of the Antiguan 
marls and cherts can be correlated with the base of the Chattahoochee limestone, 
the base of Dali’s upper Oligocene. It is also quite probable that the Oligocene 
reefs in the vicinity of Lares, Porto Rico, and of Serro Colorado, Curacao, re¬ 
present the same horizon. The Bowden, Jamaica, fauna would be slightly 
higher, to be correlated with the Chipola fauna. 
It is evident that this coral fauna from Russell Spring, besides filling a gap 
in the faunal succession on the continent, furnishes a basis for correlating many 
of the West Indian fossil reefs with the continental Tertiary section, and we 
may confidently expect more light upon the correlation of American and 
European horizons. 
1 Pumpelly, Raphael. An apparent time break between the Eocene and Chat¬ 
tahoochee Miocene in southwestern Georgia; Amer. Jour. Sci., 3rd ser., vol. xlvi, 
1893, pp. 445-447. See Vaughan, Science 1900. 
2 Vaughan, T. Wayland. A Tertiary Coral Reef near Bainbridge, Georgia. 
Science, N. S», vol. xii, 1900, pp. 873-875. 
