abstracts: geology 195 



clearly determinable. They are grouped as pre-Oligocene and post- 

 Oligocene deposits. The latter are invariably inclosed in the Miocene 

 eruptive rocks; the former in all the other rock formations. J. B. U. 



GEOLOGY. — The San Franciscan volcanic field, Arizona. Henry 

 HoLLisTER Robinson. U. S. Geological Survey Professional 

 Paper 76. Pp. 213, with maps, views, and sections. 1913. 



The San Franciscan volcanic field, which takes its name from San 

 Francisco Mountain, the largest volcano of the group, covers about 3000' 

 square miles in the north central part of Arizona. The conspicuous 

 features of the topography are the many volcanic cones rising to vari- 

 able heights up to a maximum of 12,700 feet from a northward sloping 

 plain 6000 to 7000 feet in altitude. 



The oldest rocks of the region are the pure limestone of the Redwall 

 formation, of Mississippian and Pennsylvanian age. The red sand- 

 stone of the Supai ("Lower Aubrey") formation, the crossbedded 

 Coconino ("Upper Aubrey") sandstone, and the cherty Kaibab ("Upper 

 Aubrey") limestone succeed one another in the order given and belong 

 to the Pennsylvanian series. These four formations furnish a record 

 of continuous marine sedimentation in shallow waters. 



The Moencopie formation (Permian?), consisting of red sandstones 

 and shales of fluviatile or shallow water origin, rests unconformably on 

 the Kaibab limestone and in turn is separated by a slight unconformity 

 from an overlying continental deposit of Triassic sandstone, shales, and 

 marls. These Triassic rocks furnish the last record of sedimentation 

 in this region, but a study of the surrounding country shows that deposi- 

 tion continued much longer and that Jurassic, Cretaceous, and possibly 

 Eocene strata once covered the area. 



Near the close of the Eocene epoch there was a period of folding and 

 flexing, during the Miocene one of erosion and faulting, and in early 

 Pliocene one of peneplanation. Extending from the late Pliocene to 

 late Quaternary time there were three general periods of volcanic 

 activity separated bj^ intervals of quiescence, marked by faulting, 

 uplift, and extensive erosion. The phenomena of the first volcanic 

 period were of a simple nature and consisted of widespread eruptions 

 of basalt from small cones. During the second period various lavas, 

 ranging from andesites to rhyolites, were erupted and built up a few 

 large cones. These cones, their history, their volume, together with 

 the volume of the individual lavas, and the amount of erosion since the 

 cessation of volcanic activity are described in detail. This period was 



