Reviews — H. H. Robinson — Volcanic Field, Arizona. 479 



they are getting bewildering in their profusion. Cannot a feio of 

 them be fitted together 7iow ? — thus only can we hope to understand 

 the meaning of the rest. „ ._ -,.. 



V. — The San Franciscan Volcanic Fikld, Arizona. Ey Henry 

 HoLLiSTER lloBiNSON. United states Geological Survey, Pro- 

 fessional Paper 76. pp. 213, with 14 plates aud 36 text-iigures. 

 1913. 



THE San Franciscan Tolcaiiic field, which takes its name from 

 San Francisco Mountain, the largest volcano of the group, 

 covers about 3,000 square miles in the north-central part of Arizona. 

 The State is divided into the Plateau and Basin Range provinces, and 

 the latter is subdivided into the Mountain and Plains districts. The 

 topography of these divisions is discussed and an account is given of 

 the particular features of the volcanic field, which is situated in the 

 Plateau province. 



The description of the drainage of the area is of much interest, 

 especially the examples of the changes caused by the damming of 

 watercourses by lava-flows. A chapter is devoted to the sedimentary 

 rocks, the structure, and the glaciation and alluvial deposits of 

 San Francisco Mountain. The oldest rocks of the region are of 

 Mississippian (Lower Carboniferous) and Pennsylvanian (Upper 

 Carboniferous) age, and furnish a record of continuous marine 

 sedimentation. The Moencopie formation, which is probably of 

 Permian age, rests on a somewhat eroded surface of the older rocks, 

 and consists of fluviatile or shallow-water deposits. Then folloAV 

 Triassic sandstones, shales, and marls which show evidence of con- 

 tinental deposition. No traces are left of the Jurassic, Cretaceous, 

 and possibly Eocene strata which once covered the area. 



The phenomena of the volcanic region are dealt with at length. 

 Three general periods of volcanic activity, separated by intervals of 

 quiescence are recognized. The phenomena of the first period were 

 of a simple nature, and consisted of widespread eruptions of basalt 

 from small cones. During the second pei'iod various lavas, ranging 

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

 cones. This period was further marked by laccolitic and senii- 

 laccolitic intrusions contemporaneous with the volcanic extrusions. 

 The third period closely resembled the first ia that it witnessed the 

 eruption of a single lava, again a basalt, but it was characterized by 

 the formation of a larger number of cones and a less widespread 

 distribution of the lava. Each period receives separate treatment, 

 and the details of the character, extent, and conditions of extrusion 

 of the erupted material are carefully worked out and illustrated by 

 maps and photographs of the principal cones. 



There is an interesting chapter on the history of the volcanic field 

 and adjacent country. This is followed by chapters on the petrography 

 and petrology, in which the detailed description and classification of 

 the igneous "rocks and the differentiation and composition of the 

 magma are dealt with. Analyses are given of all the important 

 rock types. 



