SECOND ANNUAL REPORT—STRATIGRAPHIC GEOLOGY. 
100 
with the friable character of the rock, makes it very difficult to obtain 
satisfactory collections. However, enough material has been obtained 
to indicate the Miocene age of the rock. Unlike the Choctawhatchee 
marl, the Jacksonville formation appears to contain practically no 
mica. It also differs from the marl in its relatively higher percentage 
of lime, and a correspondingly lower percentage of sand. 
Although the Jacksonville formation is fossiliferous, the organic 
remains are less numerous and in a much poorer state of preservation 
than in the Choctawhatchee marl. An examination of well samples 
shows that limestone forms only a minor part of the formation, a fact 
which is well illustrated by the log of the well at Jacksonville, where 
the formation attains a thickness- of about 500 feet and is composed 
largely of arenaceous and siliceous beds. From samples obtained in 
drilling a well at Jacksonville the clays are known, to be siliceous and 
the hard materials described as gravel found to be chert nodules. 
Some of the beds consist of a hard, gray, siliceous rock which appears 
to have been formed by the replacement of the calcareous portion of 
a sandy limestone by silica which was probably derived from organic 
remains such as sponge spicules and diatoms. A- detailed log of the 
Jacksonville well, supplied by Supt. R. N. Ellis, is given below: 
Feet. 
Filled ground and sand... 0-15 
Varicolored gray to red sand. 15 - 34 
Gravel .. 34 - 34^ 
Yellowish fossiliferous rock .. .. 35 - 40 
Gravel with water ... 40 - 44 
Gray fossiliferous rock.. 44 - 53 
Clay with thin layers of white rock.. . 53 - 58 
Blue clay with black gravel at 58-70 and 82-89 feet.... 58 - 89 
Rock _. ... 89 - 94 
Blue clay with black gravel. 94 -100 
Rock, 2 inches thick ...... 100 
Blue clay with quartz sand and very fine black gravel. 100 -130 
Very hard compact clay and sand.. 130 -142 
Greenish sandy clay .. 142 -204 
Greenish sandy clay with more or less black gravel and occasional 
streaks of pure clay .. 204 -250 
Sticky clay with sand and fine gravel..... 250 *-263 
Rock ... 263 -263 U 
Greenish sandy clay with heavy gravel bed ... 263^4-272 
Blue clay containing very fine sand... 272 -287 
Sand rock ...... 287 -289 
Bed of shells, oysters, etc., living types ... 289 -290 
White clay ...... ..,.. 290 -294 
Sand with clay enough to hold it... ..... 294 -298 
Compact greenish sandy clay with streaks of nearly pure clay. 298 -314 
Sand containing shells with just enough clay to hold them. 314 -320 
Shells with scraps of fossil bone..’.. 320 -330 
