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



definite information in regard to tbe limits of the different 

 formations could have been obtained. To a moderate degree 

 the character of the topography may be taken as a guide and 

 the known distribution of the different sorts of rocks thus 

 slightly extended. This applies more especially to areas of 

 sedimentary rocks as contrasted with those of igneous forma- 

 tions. 



Obispo breccias. — In the vicinity of Matachin Obispo brec- 

 cias form the hills north of the Chagres, but the surface 

 extension of the formation in this direction can not be great, 

 for, not more than four miles northwest of Matachin, younger 

 sediments are exposed in the neighborhood of Tabernilla and 

 Frijoles. From Matachin southeasterly the breccias occur on 

 both sides of the canal, in places covered by the Culebra beds, 

 beneath which they eventually disappear between Empire and 

 Culebra ; they again make their appearance in the vicinity of 

 Corozal and they have been noted at several places in the hills 

 traversed by the old Cruces trail northeast of Miraflores and 

 Pedro Miguel. The breccias are particularly well shown in 

 the rolling country northeast of the city of Panama near Las 

 Sabanas. From borings, the Obispo is known to occur as far 

 north as San Pablo, but beyond that point there is no evidence 

 of its existence. 



Basic intrusives. — Although the basic intrusives are not 

 limited to any particular part of the isthmus, they are most 

 abundant in the areas characterized by the Obispo breccias. 

 In the Culebra region they are especially numerous, and are 

 believed to make up a large part of the mountainous region 

 southwest of the city of Panama along the Pacific coast ; near 

 the shore are many exposures of columnar basalt that have 

 been observed beyond the mouth of the Chorrera Piver, 18 

 miles southwest of La Boca. !No basic intrusives have been 

 found northwest of San Pablo in the Canal Zone, but at Porto 

 Bello, about 18 miles northeast of Colon, pyroxene-andesites . 

 are exposed at the water's edge and occur in many of the 

 near-by hills. 



Acid eruptives. — The rhyolitic rocks and others related to 

 them occur in two general areas. One of these is at Panama, 

 where the well-bedded tuffs surround the central mass of 

 rhyolite porphyry of Ancon Hill. The f ragmen tal rocks 

 underlie the city and extend as far north as Miraflores. Sim- 

 ilar rocks are also found on the southwest side of the Pio 

 Grande opposite La Boca, while massive rock like that of 

 Ancon Hill composes the islands of JN"aos and Culebra in 

 Panama Bay. Hershey has reported similar rocks more than 

 one hundred miles to the southwest near Santiago.* The 



* The Geology of the Central Portion of the Isthmus of Panama, Oscar H. 

 Hershey, Bull. Dept. of Geol., Univ. of Cal., vol. ii, p. 244, 1901. 



