228 E. Howe — Geology of the Isthmus of Panama. 



teristic species Orbitoides fortisi, are of Lower Oligocene age 

 corresponding to the Vicksburg. 



Monkey Hill Formation. 



( Upper Oligocene.) 



Between Gatun and the coast at Colon is a more or less hilly 

 region in which numerous exposures may be found along the 

 lines of the Panama Railroad and the French canal. Fossils 

 collected from the rocks at these points indicate that all of the 

 beds are of the same age. The rocks are well stratified, often 

 thin bedded, calcareous sandstones, argillaceous sandstones, 

 marls and shales, usually fine grained and even-textured, and 

 when fresh of a neutral bluish or greenish gray color. They 

 belong to what Hill called the Monkey Hill beds,* the best 

 exposures at the time of his visit being near the Panama Rail- 

 road where it passes through the hills. At the time of my 

 examination by far the best exposures were at Gatun, where 

 extensive excavation had been made for the locks. At this 

 point also it is believed that the base of the formation is shown. 

 In order to avoid ambiguity these beds will be referred to as 

 the Monkey Hill formation, as they were so described by Hill, 

 although in my report to the Canal Commission they were 

 spoken of as the Gatun beds, since they made up a large part 

 of the rocks through which the locks at Gatun were to be con- 

 structed. 



From fossils collected both by Hill and myself at Monkey 

 Hill and Gatun, Dr. Dall considers that the Monkey Hill for- 

 mation is equivalent to the Chipola Oligocene, that is, younger 

 than the Yicksburg or the Orbitoides marls of Peiia Blanca. 

 Among the species recognized are : 



Cardium sp. Turritella 

 Piocardium serratwn Linn. Oliva sp. 



Psarnmobia Gadulus 



Cytherea Cerithiopsis 



Abra Agriopoma 



Tellina Cyclinella. 

 Chione 



Directly below and within a foot or two of the point where 

 these fossils were collected at Gatun is a coarse conglomerate 

 also containing numerous fossils that are pronounced by Dall 

 as of the same age as those found near Gatun in the Bohio 

 beds. The conglomerate appears to grade upward into the 

 fine calcareous sandstone or marl that contains the younger 

 fossils, and were it not for the definite indication of greater 

 *Op. cit. s 176-177, 208. 



