60 H. E. Gregory — J. Geologic Eeconnaissance 



crysts and as a constituent of the groundmass; brown horn- 

 blende, in many places resorbed; iron ore, chiefly magnetite, 

 distributed as grains and also replacing biotite and hornblende ; 

 titanite and apatite in small quantity. Calcite, serpentine, 

 chlorite, chalcedony, and quartz constitute the list of secondary 

 minerals. One of the specimens collected proved to be an 

 andesitic tuff, or tuffaceous sandstone. It consists of well- 

 rounded grains of clear quartz, partly decomposed fragments 

 of microcline and oligoclase, and lapilli of andesite, all firmly 

 cemented by silica. In comparison with other fragments the 

 chunks of andesite are large and angular and suggest contem- 

 poraneous igneous activity at no great distance. 



In texture the individual strata of the Pachatucsa formation 

 vary widely and abruptly, both along the strike and in the 

 direction of dip. ]STo beds of fine ash were noted. Large 

 blocks of lava are common and at one locality a mass of 

 amygdaloidal andesite 80 feet long and 30 feet thick was 

 observed. The size of these masses, their conformable relation 

 to underlying beds, and the fact that vesicles on their surfaces 

 as well as larger cavities and joints are occupied by sandstone 

 suggest contemporaneous extrusions. It is not improbable that 

 further study will reveal the presence of lava flows as persistent 

 members of the stratigraphic series. 



In the Atasccasa region the presence of igneous knobs and 

 the wide variation in thickness and extent of the lenslike beds 

 have produced a topography of hills and flats entirely unlike 

 the Pachatucsa cuesta. In this area the strata of the Pacha- 

 tucsa formation are cut by intrusions and their volcanic con- 

 stituents are apparently nearer their source. Volcanic ash in 

 beds and lenses 2 to 90 feet thick is interstratified with volcanic 

 conglomerate and sandstone. ]STear the intrusive masses the 

 arenaceous sediments have been converted into quartzite, and 

 beds of volcanic breccia near one of the dikes are believed to 

 indicate the site of an ancient vent which supplied at least part 

 of the igneous fragments present. In addition to igneous 

 fragments the conglomerate and ash beds at Atasccasa contain 

 innumerable rounded and subangular quartz fragments as 

 much as half an inch in diameter, also feldspars and blocks 

 of sandstone, shale, and quartzite. 



The Pachatucsa beds are believed to be of continental origin, 

 but their age is undetermined. The strata seem to underlie 

 conformably the red sandstones and shales of the Huayllabamba 

 formation at the ede;e of the overlving Ichchu-Orceo lava and 



