96 II. E. Gregory — A Geologic Reconnaissance 



list of secondary minerals. This andesitic tuff is believed to 

 be genetically related to the basic andesite over which it lies. 



Oropesa. 



Between Oropesa and Huacarpay the Rio ITuatanay follows 

 the cliff front of an igneous mass nearly 2 miles in length. 

 No crater and no well-defined flows were seen during my brief 

 examination. .The surface of the mass, however, is covered 

 with irregular blocks of lava standing at various angles, and 

 along the railroad near Huacarpay ropy amygdaloidal lava 

 clearly indicates the extrusive character of at least a part of 

 the mass. As viewed in the hand specimen the fresh rock is 

 dark gray and dense except for innumerable specks of 

 biotite and rounded bits of calcite occupying amygdaloidal 

 cavities. Weathered specimens are light gray with micas of 

 bronze color. The rock has the surface appearance of a 

 scoriaceous minette. 



The constituent minerals of the Oropesa lava as revealed by 

 the microscope are biotite, hornblende, augite, plagioclase, iron 

 ore, and calcite. The biotite occurs in long brown, strongly 

 pleochroic plates, as phenoerysts and as smaller shreds in the 

 groundmass. The hornblende is also brown and pleochroic, 

 closely resembling the biotite, from which it differs in crystal 

 outline. Augite occurs in colorless prismatic and elongate sec- 

 tions, rarely twinned. The plagioclase, which presents both 

 laths and tabular forms, was found to be predominately 

 labradorite. Grains of iron ore are few in number and widely 

 scattered. The essential constituents are labradorite, biotite, 

 and augite ; hornblende and iron ore are accessary. The tex- 

 ture is hyalopilitic — small laths of feldspar embedded in a 

 matrix of glass. Included in the lava are fragments of gray 

 granite, garnetiferous gneiss, amphibolite, quartzite, and gar- 

 netiferous mica schist. In accordance with the classification 

 in general use the Oropesa lava may be termed a basalt or 

 mica basalt. 



Sierra-Bella. 



At Sierra-Bella, within the canyon of the Ilrubamba, 

 the remnant of a lava flow forms a terrace rising 150 

 feet above the river (fig. 42). The terrace front is a wall 

 formed of well-developed columns. On the opposite side of 

 the river, dikes of intrusive rock, essentially like the columnar 

 lava in composition, cut the country rock. It is possible that 



