712 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



is of a peculiar brownish color, unlike that of the Obispo breccia; the fragments are apparently 

 angular and usually less than 1 inch in diameter. One of the most striking features of this 

 rock, shown in deep borings as well as at surface outcrops, is its alteration; it mashes under a 

 blow of the hammer and yet has not the appearance of being greatly decomposed, except at 

 or very near the surface. The alteration seems to have been to serpentine or some related 

 mineral instead of kaolin, as is usually the case with the other rocks, especially those of the 

 Obispo formation. 



Across the river, at the old lock site south of Bohio and east of Pena Blanca, quite a 

 different phase of this formation is exposed ; it consists of alternating layers of sandstones and 

 coarse conglomerates all clearly water transported and deposited. Many of the bowlders 

 are a foot or more in diameter; they are of andesites for the most part, but not a few are of a 

 lighter-colored porphyritic rock with abundant hornblende crystals. The finer layers and 

 the cementing material are very like the rock occurring at the Bohio quarries. Cross-bedding is 

 common and fine material often occurs in lenses in the grits and conglomerates. The prevailing 

 strike i^ N. 25° E., and the dip to the northwest at an angle of about 14°. 



The identity of these conglomerates and the more even-textured breccia-like rock is not at 

 all evident in the field, but many borings made in the vicinity show a gradual transition south- 

 ward from the even-textured breccias of Bohio to the conglomeratic facies at Pena Blanca. 



The boring made at kilometer 24 (hole 24-b) is of interest, since it shows carbonaceous 

 sandstones in the middle part and fossils in the lower part of the core. 



Further study of tliis formation to the southeast and by means of borings has shown that 

 it is of variable character, in some places clearly sedimentary and water sorted, both fine grained 

 and conglomeratic; in other places the appearance is more that of a volcanic breccia, possibly 

 water-laid but not water transported. * * * 



In general the formation may be described as a probable fluviatile deposit characterized 

 by sudden changes and transitions, the composition of the materials being fairly uniform. 

 It is not improbable that volcanic eruptions may have been taking place during the period 

 in which the formation was deposited and the material forming the breccia-like portion attributed 

 directly to such a source. 



The fossils previously referred to that were found in the core taken from hole 24-b occur 

 at elevations of from 30 to 40 feet below sea level. They include, in addition to vegetable 

 matter, certain fossils indicating probable Eocene age. At about 2 miles to the west, at a locality 

 known as Vamos Vaihos, on the left bank of the French canal, are outcrops of a limy shale 

 with large calcareous , concretions resembling bowlders in a conglomerate; both the shale and 

 concretions contain numerous fossils which Dr. William H. Dall regards as of undoubted Eocene 

 age. Outcrops of similar rock containing the same fossils occur about 6 miles to the northwest 

 on the left bank of the canal in the vicinity of Gatun. 



The rocks at Vamos Vamos and in the vicinity of Gatun are finer grained than those of the 

 Bohio formation and consist of shales and fine sandstones. 



The fossil evidence shows with certainty that the beds near Gatun and at ^'amos Vamos 

 and Bohio are of the same age, and it is believed that they are to be regarded strictly as part 

 of one formation, the occurrence at Boliio representing a littoral facies, while those farther 

 to the northwest were deposited in deeper water. 



In the excavations made by the French between Bohio and Pena Blanca, and also at the 

 base of a little hill at the west end of the village of Bohio, there are outcrops of a white calcareous 

 sandstone or marl containing some volcanic material in the form of crystals of feldspar and 

 fine specks of a ferromagnesian mineral. In places it is extremely rich in the remains of For- 

 aminifera, especially OrMtoides fortisi, and may seem to consist altogether of the shells of these 

 minute organisms. * * * 



The Foraminifera of the Pena Blanca rocks, and especially the species Orhitoides fortisi, 

 are characteristic of the Lower Oligocene. From their position between two well-determined 

 occurrences of Eocene rocks it is believed that they are clearly younger than the Bohio forma- 

 tion and rest upon these rocks unconformably. 



