E. Howe — Geology of the Isthmus of Panama. 217 



ferent textures, often fluid al and glassy, and varying from a 

 tenth of an inch to one or two inches in diameter. Although 

 many of the fragments are comparatively fresh, the matrix or 

 cement is in an advanced state of decomposition, the second- 

 ary minerals being zoisite, epidote, serpentine, and some doubt- 

 ful kaolinite. As a whole, however, the rock is so extremely 

 altered that it is difficult to determine whether it is a simple 

 volcanic breccia like the Obispo or a fine conglomerate of vol- 

 canic debris like that at the near-by lock site. So far as it is 

 possible to make out, I am inclined to believe that it is a vol- 

 canic breccia, in the sense that it is more or less of eruptive 

 origin, largely on account of the uniform character of the 

 rock fragments composing it as contrasted with the varied pet- 

 rographic character of the bowlders in the conglomerate at 

 the lock site. It is not impossible that the breccia represents 

 a ''volcanic mud-flow" as suggested by Bertrand.* 



The particular interest attaching to this breccia is its inti- 

 mate association with the conglomerates exposed at the lock 

 site. This relation is not shown on the surface, but is brought 

 out very clearly by borings made in exploring possible dam 

 sites in the vicinity. One series of these borings, along what 

 is known in the surveys as the "F" line, extended from the 

 quarries at Bohio in a southerly direction across the river to 

 the hill through which the French excavated the lock site, a 

 distance of about one-half mile. At the northern and southern 

 ends of this line respectively characteristic specimens of the 

 breccia and of the conglomerate were obtained by the drill, 

 while at intermediate points transitional facies were shown in 

 nearly all the borings. Among other features is the associ- 

 ation of the typical breccia of the quarry with water-laid sand- 

 stones or tuffs, many of which contain carbonaceous matter, 

 while others, coarser grained, are composed of distinctly water- 

 worn material and are held in a calcareous cement. Passing 

 southward the breccias become less and less abundant while 

 the water-worn material increases in coarseness and conglomer- 

 atic beds are to be noted in many of the holes. Layers of fine 

 material interbedded with both the typical breccia and with 

 the coarse conglomerates, as at the French lock site, are shown 

 in practically all the borings. 



The lowest point reached by the drills was about 194 feet 

 below sea level near the middle of the line of borings. After 

 passing through alluvium filling the Pleistocene valley of the 

 Chagres, the deepest boring entered about thirty feet of coarse 

 conglomerate ; the matrix holding the bowlders was ground up 

 by the drill and none is shown in the samples collected, but 



* Op. cit., p. 5. 



Am. Jour. Sci.— Fourth Series, Vol. XXVI, No. 153.— September, 1908. 

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