808 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



This formation rests unconformably upon the Alum Bluff formation and is also separated from 

 the overlying Duplin marl by a small unconformity represented by an uneven line of quartz 

 pebbles. The strata consist of gray or brownish compact argillaceous sand containing large 

 calcareous nodules and, in places, of friable phosphatic sand containing shells. The formation 

 does not reach a thickness of more than 30 or 35 feet in natural exposures. 



"The Duplin marl as exposed along Savannah River overlies the Marks Head marl and is 

 separated from it by a small unconformity indicated by quartz pebbles. It is overlain by 

 Pliocene (Lafayette ?) and Pleistocene deposits of sand and clay. It is a shell marl consisting 

 of shells in a matrix of coarse phosphatic sand; in places it consists of a fine gray or brown quartz 

 sand. It probably does not reach a thickness of more than 10 or 12 feet. 



"The Duplin on Altamaha River consists of friable sandy and pebbly shell marl and bluish 

 fine-grained argillaceous sand, which is also fossihferous. The formation unconformably 

 overlies strata of probable Alum Bluff age, and is in turn overlain by varicolored sand and clay 

 which is probably Lafayette (Altamaha). The Duplin reaches a thickness of not over 12 or 

 15 feet. 



"The Miocene at Brunswick is not exposed naturally and was discovered in dredging opera^ 

 tions on Brunswick River. The material thrown off the dredge consists of fragments of bones, 

 teeth, sand, quartz pebbles, and compact sandy marl, or shells embedded in a phosphatic sand 

 matrix, together with some argillaceous limestone and hard clay. A study of the fossils has been 

 made by Vaughan and they were found to range from Miocene to Recent. 



"Miocene rock is exposed in the bed and banks of Satilla River at low tide, at Owens Ferry, 

 8 miles west of Woodbine. The rock is a brown or grayish compact sand and calcareous 

 sandstone, containing poorly preserved fossils." 



The Pliocene in Florida rests upon the eroded surface of the upper Oligocene Alum Bluff 

 formation, as is shown in the exposures at Jackson Bluff, on Ocklockonee River, and at Alum 

 Bluff, on Apalachicola River. The series in this State is divided into two formations that occupy 

 different areas but are probably in large part contemporaneous. The formation in the eastern 

 area is the Jacksonville, that in the western area the Choctawhatchee. Matson and Clapp^"'' 

 describe the Jacksonville as follows: 



"From well records and samples obtained along the east coast of Florida, the Jacksonville 

 formation appears to rest unconformably upon the eroded surface of the limestones of the Vicks- 

 burg group at Jacksonville, St. Augustine, and other localities. Farther westward it probably 

 rests on the beds belonging to the Apalachicola group, but no contacts were observed. 



"When fresh, the limestone of the Jacksonville formation varies in color from hght gray to 

 nearly white, but upon weathering it changes to pale yellow or yellowish gray. It usually has a 

 porous texture but occasionally becomes hard and dense. The presence of a large percentage 

 of clear quartz sand may be easily distinguished by the use of an ordinary hand lens, and micro- 

 scopic examination shows that there is also a large amount of clayey material which varies in 

 color from light gray to pale yellow. In certain horizons fossils are very abundant, the shells 

 having usually been dissolved, leaving nothing but casts or molds ; and this fact, together with 

 the friable character of the rock, makes it very difficult to obtain satisfactory collections. How- 

 ever, 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 fossihferous, 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 



