214 E. Iloioe — Geology of the Isthmus of Panama. 



Pacific side of the isthmus in the vicinity of Las Sabanas, and 

 this has been compared with specimens from the other locali- 

 ties which seem to be equivalent. 



The formation as a whole consists of moderately coarse 

 breccias of andesite of a variety of textures but nearly uniform 

 composition. Fine-grained tuffs have been observed, mostly 

 on the northern side of the isthmus, while the Sabanas occur- 

 rences present an extremely coarse facies. As a rule the 

 breccias of the interior are composed of angular fragments 

 varying from a quarter of an inch to two inches in diameter. 

 At a quarry near the Sabanas road one block more than six 

 feet in diameter and many twelve or eighteen inches in diam- 

 eter were observed in a cement as coarse as the average breccia 

 of the interior. In a region where outcrops are few and con- 

 tinuous exposures unknown it is not possible to make general- 

 izations, but from the observed occurrences it may be said that 

 the usual appearance of the Obispo breccia lies between the 

 extremes of coarseness and fineness that have been mentioned. 



So far as observations have been carried, the rock fragments 

 composing the breccias consist largely if not entirely of 

 pyroxene-andesite. Some are highly vesicular and glassy, 

 others markedly porphyritic, while the majority are dense, 

 even-textured rocks in which individual crystals can be made 

 out only with the aid of a lens. In one specimen there were 

 doubtful indications of hornblende. The prevailing greenish 

 gray color of the rock suggests what is found to be the case on 

 microscopical examination, that the pyroxene, usually augite, 

 has undergone extensive alteration; on the other hand, the 

 plagioclase, which generally has a composition of about 

 Ab 2 An 3 , is comparatively fresh, a fact noted in the cases of 

 most pf the rocks of the isthmus where the final stage in the 

 decomposition to the surface red clay has not set in. 



Distribution of the Obispo breceia.- — The most northerly 

 occurrences of these breccias are shown by borings at the San 

 Pablo dam site ; from that point southward they appear occa- 

 sionally at the surface, and are indicated in all the borings 

 made at intervals of one kilometer along the center line of the 

 canal as far as Empire. Decomposed but otherwise character- 

 istic exposures are to be seen between Mamei and Gorgona 

 along the Panama Railroad, and from G-orgona to Empire, 

 where they disappear beneath a cover of later sediments, the 

 Obispo breccias are the prevailing rocks. Composing the hills 

 north, west, and east of Corozal, and particularly well exposed 

 northeast of Panama in the vicinity of Las Sabanas, are rocks 

 of the same petrographical character that are believed to be a 

 part of the central mass. 



