E. Howe — Geology of the Isthmus of Panama. 229 



age from the fossil evidence, I should be inclined to consider 

 this conglomerate a basal member of the Monkey Hill forma- 

 tion ; it is, of course, not impossible that this may be the case 

 and that the fossils were derived from the erosion of the older 

 Bohio terrane. Less than half a mile in a straight line from 

 this locality is the point where the Bohio (Claibornian) fossils 

 were collected at the edge of the French canal near Gatun 

 (Kilometer 10), and the general dip of the beds at this locality 

 is in the direction of the conglomerates exposed at Gatun. 

 In any event the base of the Monkey Hill formation must be 

 very near the conglomerate, whether actually at the top or 

 including the conglomerate. Unfortunately there are no 

 other exposures in the vicinity to throw light on this matter 

 and little can be learned from the drill records. The conglom- 

 erate is clearly shown by a number of borings, below which 

 are beds of a white pumiceous tuff associated with other con- 

 glomerates similar to the uppermost one, and calcareous sand- 

 stones and marls. Beneath these beds are fine calcareous or 

 argillaceous sandstones of uniform composition that undoubt- 

 edly belong to the Bohio formation. 



At Gatun about eighty feet of the Monkey Hill formation 

 is shown, but, as in the case of the other sediments, it is only 

 possible to give an approximate estimate of the thickness of 

 the whole formation. From rough calculations, assuming low 

 dips of from one to five degrees from Gatun northward, there 

 should be a thickness of at least five hundred feet in the vicin- 

 ity of the Monkey Hills near Colon, while perhaps from two 

 to three times this thickness may exist. To the west, where 

 the Chagres leaves its main valley and passes through a younger 

 valley to the sea, a thickness of from one hundred and fifty to 

 two hundred feet is actually shown in the hills, while as much 

 more must exist beneath the present noodplain of the river if 

 the dips observed in the region are at all regular. 



No rocks corresponding in age to those of the Monkey Hill 

 formation have been observed on the Pacific side of the isth- 

 mus. 



The lithologic character of these rocks is essentially the 

 same as that of the finer-grained Bohio rocks. They are com- 

 posed almost entirely of the debris of igneous rocks, in some 

 cases the particles being exceedingly fresh, but commonly 

 decomposition and fineness of texture make it impossible to 

 say whether the material was derived directly from volcanic 

 eruption or from the degradation of an older land surface. I 

 am in favor of the latter hypothesis, inasmuch as no evidence 

 has been found elsewhere of contemporaneous eruptions. 



