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



the presence of certain porphyritic rocks closely resembling 

 those contained in the conglomerate of the lock site makes me 

 believe that this is to be regarded as a conglomerate rather 

 than as a breccia like that of the quarry where little or no vari- 

 ation in the character of the fragments of pyroxene-andesite 

 was found. One hundred feet south of this point another 

 boring passed through fine sandstones or tuffs before striking 

 beds similar to those in the previous hole and at a correspond- 

 ing elevation. About the same distance to the north from the 

 first hole and at the same elevation, tine-grained beds like those 

 of the second hole are also shown by the core, while still nearer 

 the quarries and at an interval again of about one hundred feet 

 fine material mixed with angular fragments of pyroxene and 

 andesite is shown. In the opposite direction a corresponding 

 change to the conglomerate facies is to be observed. Precisely 

 the same transition is shown by a line of borings made from 

 the hill in which the French lock site was excavated in a north- 

 easterly direction across the valley of the Chagres to the hills 

 east of Bohio. 



This evidence would seem to indicate that the breccias, best 

 shown at the quarries, were deposited contemporaneously with 

 the conglomerates and sandstones of the lock site. The breccias 

 were in large part, if not entirely, laid down in water, and prob- 

 ably running water, but it is impossible to decide whether the 

 material composing the breccia was transported for some dis- 

 tance by streams before being deposited or if it fell into the 

 water directly from the air. The angular character of the 

 fragments, so far as it may be determined in the extremely 

 decomposed rock, would seem to favor the direct deposition of 

 the breccia in water as a result of volcanic eruption. Eruptions 

 of fragmental material of uniform composition undoubtedly 

 took place during this first period sedimentation, and evidence 

 for this has been found at other points, especially in the vicin- 

 ity of Culebra, as stated in a later paragraph. 



Southeast of Bohio the character and distribution of the 

 conglomerates and breccias are imperfectly known. Near San 

 Pablo rocks similar to the Bohio breccias are indicated by bor- 

 ings beneath acid tuffs, while it is more than probable that the 

 Bohio beds are represented in the neighborhood of Gorgona 

 by conglomerates exposed along the line of the Panama Pail- 

 road. In most instances it is all but impossible to distinguish 

 between the conglomerates and the volcanic breccias upon 

 which they rest because of the extreme decomposition that 

 both have suffered. The occurrences at Gorgona are fresher, 

 however, and their conglomeratic character unmistakable. 

 The andesitic breccias beneath these conglomerates belong to 

 the Obispo formation and have been traced by borings practi- 



