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



raagnesian mineral, altered beyond recognition, has stained the 

 rock slightly with iron oxide ; there is no indication that the 

 dark silicate was an important constituent. Another specimen 

 of the same rock from a near-by locality showed little or no 

 porphyritic texture.; a few fragments of orthoclase and laths 

 of oligoclase are present, with many large irregular patches of 

 quartz ; there is some magnetite and limonite but apparently 

 no ferro-magnesian mineral. 



The islands of Xaos and Culebra in Panama Bay, between 

 three and four miles from Ancon Hill, are composed of a similar 

 porphyry containing a moderate number of phenocrysts of 

 plagioclase ranging from anclesine to a labradorite (Ab 2 An 3 ), 

 and hornblende. The groundmass is noncrystalline and con- 

 sists of orthoclase and plagioclase in about equal amount with 

 traces of hornblende, usually altered to chlorite, some magnet- 

 ite, and quartz. 



Closely related to the rock of Xaos Island is an intrusive 

 sill found in the beds at Culebra north of Gold Hill (Cerro 

 Culebra). This rock is a fine-grained porphyry with a fluidal 

 base, largely glassy, but containing a few microlites of plagio- 

 clase and minute grains of orthoclase. The phenocrysts are 

 mostly andesine although labradorite is present. A dark min- 

 eral, perhaps hornblende, was observed in several specimens, 

 but in all cases it has been considerably altered. In one speci- 

 men it was impossible to decide whether the altered mineral 

 had been hornblende or biotite, the ragged form suggesting 

 the latter. Near Las Cascadas, and associated with the Obispo 

 breccia, a dense rock with a marked fluidal texture is exposed 

 at several places. Under the microscope it is found to consist 

 of a partly glassy groundmass, laths of plagioclase, a little 

 orthoclase and an abundance of fine grains of magnetite. The 

 few plagioclase phenocrysts are considerably decomposed and 

 ferro-magnesian minerals are notably absent. 



All of these rocks, with the exception of the Ancon porphyry, 

 appear to be intermediate between true rhyolites and latites, 

 with perhaps a stronger tendency toward the latitic form ; the 

 impossibility, however, of determining the exact character of 

 the groundmass in most cases prevents a definite classification 

 by microscopical methods, and none of the specimens, as has 

 been said, was sufficiently fresh to warrant chemical analysis. 

 The tuff exposed by the Chagres River in the vicinity of San 

 Pablo consists of small fragments of fibrous or vesicular glass 

 or pumice of low specific gravity ; there are occasional par- 

 ticles of plagioclase and orthoclase crystals but no quartz. A 

 greater textural variation is found in the tuffs in the vicinity 

 of Panama ; they are less pumiceous and consist of fragments 

 of rock essentially the same as that of Ancon Hill. 



