94 II. E. Gregory — A Geologic Reconnaissance 



greenish tone and the ortho-rhombic variety hypersthene in 

 irregular grains and prisms of green to brown color. The 

 larger crystalline fragments of hypersthene are closely associ- 

 ated with the mica and in some places appear as poikilitic 

 inclusions within spongy biotite crystals. Apatite occurs in 

 prismatic and basal crystal sections; grains of iron ore are 

 widely scattered. The rock may therefore be called hypersthene 

 andesite. 



Atasccasa. 



The geology of the Atasccasa area is too complex to admit 

 of a satisfactory interpretation by a reconnaissance survey. 

 Contacts are concealed for the most part by glacial drift, the 

 rocks are faulted and brecciated, and the sediments intruded 

 by igneous masses are themselves in large part composed of 

 igneous material. This area has, therefore, been mapped in 

 a more or less arbitrary fashion, and only the more prominent 

 igneous outcrops are indicated. Ancient intrusive masses 

 which contributed materials for the Pachatucsa formation are 

 probably represented, together with dikes of later age. Extru- 

 sive rocks contemporaneous with or later in age than the 

 Pachatucsa beds are doubtless also present. Exclusive of 

 fragments, many of them tons in weight, incorporated in sand- 

 stone and tuff, the igneous rocks examined from this area 

 belong to the trachyte and the andesite families. 



The westernmost outcrop shown on the map is a low ridge 

 with cliffs on its southern face and broken by prominent joints 

 trending ]ST. 60° W. The rock is imperfectly gneissoid in 

 structure and pink in tone. Under the microscope it is seen 

 to consist essentially of much-altered pink orthoclase in pris- 

 moid and tabular forms. Albite is also present, and apatite 

 occurs with characteristic features. Chlorite, probably derived 

 from the cryptocrystalline matrix, is scattered throughout the 

 section, and glass is present in small amounts. The texture 

 appears to be hyalopilitic. The rock may be classed with 

 trachytes or syenites. 



As seen in the ledge the three other areas of igneous rock 

 outlined on the geologic map include massive, bedded, brecci- 

 ated, and conglomeratic facies. At one point an agglomerate 

 of igneous fragments incorporated with masses of quartzite 

 and sandstone suggests the site of a volcanic pipe. Thin sec- 

 tions cut from specimens collected from the larger igneous 

 masses within the three areas under discussion contain plagio- 



