E. Howe — Geology of the Isthmus of Panama. 221 



Protocardia, and Ostrea, a small Fusus (?), a specimen of Or- 

 oulina, and &Melanian (?) associated with abundant fragmentary 

 plant remains. The abundance of carbonaceous material and 

 the presence of the fresh-water Melanian within a few feet 

 vertically of the salt or brackish water species suggest delta 

 deposits, and with this the physical character of the beds agrees 

 perfectly, the fossils being preserved in calcareous sandstones 

 or shales between beds of conglomerate precisely as are those 

 exposed at the cutting for the lock site. Before reaching these 

 fossiliferons layers the drill passed through a fine breccia like 

 that shown at the quarries at Bohio, interbedded with which 

 are fine-grained sandy layers containing plant remains. Con- 

 glomerates like those of the lock site were found below this 

 breccia, then at about twelve feet below sea level a fine car- 

 bonaceous sandstone layer was encountered, and below this the 

 fossiliferons sandstones. 



On the strength of this evidence I am inclined to regard the 

 conglomerates and breccias at Bohio as members, probably 

 occurring near the base, of the formation that 1 have called 

 the Bohio, and that they were laid down as delta deposits at 

 the mouth of a large river, while the Tamos Yamos beds and 

 those near Gatun were deposited contemporaneously in deeper 

 water. It is entirely possible, of course, that the conglomerates 

 may belong to an earlier epoch than the Claiborne, but the 

 evidence for or against this is so meagre that in the absence of 

 anything to the contrary it seems entirely reasonable to place 

 the conglomerates at the base of the Claibornian Bohio forma- 

 tion. Xo one can doubt the intimate relation between the 

 conglomerates and the breccias nor that they are contempora- 

 neous. It has been shown also that the conglomerate occur- 

 ring in patches near Mamei and Gorgona rests unconformably 

 upon the breccias of the Obispo formation. For this reason 

 and as is also shown in the case of the Culebra beds, I believe 

 that the Bohio rocks with their breccias are younger than the 

 breccias of the Obispo formation instead of being a part of 

 them as suggested by Bertrand. 



Thickness of the Bohio Formation. — Of the thickness of 

 the Bohio formation very little can be said. Borings on the 

 •* F" line at the Bohio dam site have brought up samples of 

 the conglomerate, sandstone and breccia from nearly 200 feet 

 below sea level, while at least 75 feet more may be added to 

 this as exposed above sea level in the near-by hills. From 

 Bohio to Gatun, where the Bohio formation is covered by 

 younger beds, the distance is about seven miles in a straight 

 line ; assuming a uniform dip of only two degrees to the north- 

 west, this would give at Gatun a thickness of nearly 1300 feet. 

 At least 300 feet of the section has been shown in borings at 

 Gatun. 



