SECOND ANNUAL REPORT-STRATIGRAPHIC GEOEQGY. 
HI 
Concerning the thickness of the Miocene in Florida, Dali 1 says: 
The Chesapeake group is represented over a very wide area in Florida, if 
the scattered observations already made can be regarded as indicative of its 
extension. Borings on the eastern coast of Florida and in the St. Johns valley 
indicate that there the beds of this group in some places attain a thickness of 
at least 500 feet. 
Some uncertainty arises when thickness is estimated from samples of rock 
obtained from borings, and the difficulty is increased several fold when it is 
necessary to rely upon descriptions prepared by drillers. 
The information relating to the thickness of the Jacksonville for¬ 
mation at various points on the east coast will be discussed in con¬ 
nection with the detailed sections; and it is only necessary here to 
note that Dali’s estimate of the thickness of the formation is probably 
essentially correct. Samples obtained from a well at Jacksonville 
indicate that at that locality the formation has a thickness of over 
460 feet. This is shown by the fact that the limestone phase of this 
formation was encountered at a depth of thirty-five feet; and there 
was obtained at a depth of 495 feet a shark’s tooth which does not 
occur in rocks older than the Miocene. 
Physiographic Expression: — Where the Jacksonville formation 
lies near the surface characteristic sink-hole topography is sometimes 
seen. This is the case at the original Miocene locality near Rock 
Spring; but here, as in some other localities, it is impossible to say 
how much of the topography is due to the underlying porous limestone 
of Oligocene age. 
Paleontologic Characters:—Owing to the difficulty of obtaining 
good collections, the fauna of the Jacksonville formation is imper¬ 
fectly known. Dali 2 reports fossils from various localities, among 
them being Pecten jeffersonius, Carditamera arata, etc., from Preston 
sink three miles north of Waldo; and Venus rileyi, V. permagna and 
Area limula in a well at St. Augustine at a depth of 208 feet. 
Heilprin 3 identified Pecten madisonius, Venus alveata, V eneri- 
cardia granulata, Carditamera arata, Mytiloconcha incurva, Cardium 
sublineatum and Oliva literata from Rock Springs, Orange County. 
During the progress of the recent field work a few fossils were ob¬ 
tained from exposures on Black Creek. While the collections were 
small, it was possible for Dr. Vaughan to determine a few species. 
The lists of these are given on subsequent pages. 
Structure:—The Jacksonville formation presents no peculiarities 
of structure, and appears to have undergone no considerable disturb- 
1 Dali, Wm. H. Neocene of North America, U. S. Geol. Survey, Bull. 84, 
1892, p. 124. 
2 Dali, Wm. H. Neocene of North America, U. S. Geol. Survey Bull. 84, 
1892, pp. 124-125. 
3 Smith, E. A., Am. Jour. Sci., 3rd ser., vol. xxi, 1881, p. 302. 
