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



Igneous Rocks. 

 Acid Porphyries and Tvffs. 



The oldest igneous rocks, the breccias of the Obispo forma- 

 tion, have already been described, and it has been shown that 

 eruptions of much the same character continued intermittently 

 for some time during the period of early Tertiary sedimenta- 

 tion. After an interval of quiet, eruptions of an entirely dif- 

 ferent sort of rock took place either at the close of the Bohio 

 epoch or at the beginning of the Monkey Hill. These later 

 eruptives are largely f ragmen tal and are found on both the 

 Atlantic and Pacific sides of the isthmus. The massive rocks 

 of this period are best shown at Ancon Hill, which is com- 

 posed entirely of rhyolite porphyry. Almost completely sur- 

 rounding Ancon Hill, the continuity being broken only on the 

 west by a later intrusion of pyroxene-andesite, are tuffs and 

 fine breccias of the same composition as the rhyolite porphyry 

 of the hill. The tuffs are well bedded and in some places 

 quite massive, individual beds varying from six inches to five 

 or six feet in thickness ; they are fine-grained and, where 

 exposed near the surface, usually altered to a white clay. 



Beds similar to those in the vicinity of Panama are exposed 

 between San Pablo and Tabernilla on the Atlantic slope, the 

 best outcrops being along the Chagres Biver near the point 

 where the Panama Pailroad crosses northwest of San Pablo. 



These comparatively siliceous rocks are the ones that Hill 

 placed in his Panama formation ; he referred to them as rhyo- 

 litic tuffs,* while Bertrand considered them as trachytic.f 

 Specimens that I collected from various points showed greater 

 or less decomposition in most cases and none was sufficiently 

 fresh for chemical analysis. A microscopical study shows 

 that they are all closely related and that in addition to the 

 occurrences mentioned, certain intrusives in the Culebra beds 

 probably belong to the same series. It is impossible to group 

 all these rocks under one descriptive head such as rhyolite 

 tuff or trachyte tuff; certain facies are distinctly rhyolitic, 

 others trachytic, while forms near quartz-bearing latite are not 

 uncommon. 



The rock of which Ancon Hill is composed is a creamy- 

 white porphyry with phenocrysts of feldspar in a fine felsitic 

 ground mass ; occasional specks of an altered dark mineral are 

 present. Microscopically the rock is seen to be a porphyry 

 with phenocrysts (mentioned in order of relative abundance) 

 of albite, quartz, and orthoclase in a groundmass containing 

 abundant orthoclase, some quartz and a little albite. Slight 

 kaolinization of the feldspars has taken place, and the ferro- 

 *Hill, op. cit., 199-202. f Bertrand, op. cit., 9, 23. 



