202 Scientific Intelligence. 



3. The San Franciscan Volcanic Field ; by Henry Hollister 

 Robinson ; Professional Paper No. 76, U. S. Geo!. Surv., 8vo, 

 pp. 213, maps and figs., 1913. — The San Francisco Mountains, so- 

 called, in Arizona have in recent years become well known as 

 they are situated beside the tourist route to the Grand Canyon of 

 the Colorado. In addition to the prominent central mass of San 

 Francisco Mountain there are a number of other peaks situated in 

 its vicinity or at no great distance from it, the whole constituting 

 a distinct field of volcanic activity on this portion of the great 

 plateau. It is of this volcanic group that Dr. Robinson here pre- 

 sents the results of extensive studies in the field and of the mate- 

 rial collected by him in the laboratory. The result forms one of 

 the most detailed investigations of an extinct volcanic group 

 which has yet been undertaken in this country. 



After the introduction and a general consideration of this 

 region, the author gives an account of the sedimentary platform 

 upon which the volcanoes have been built, and then takes up the 

 general geology of the volcanoes and lava fields. He shows that 

 there were three general periods of volcanic activity, beginning 

 with outflows of basalts, followed by the formation of the volcanic 

 cones, and ending with another outbreak of basalt, forming a 

 great number of small cones and of flows. 



The second period presents the most complex stage of volcanic 

 activity, as the cones in general are built up of various kinds of 

 lava. Thus at San Francisco Mountain five stages of eruption, 

 each yielding a different type of lava, have been worked out. 

 The geology of each of the main peaks is described in detail, first 

 with San Francisco Mountain, the main cone of the district, fol- 

 lowed by Kendrick Peak, Bill Williams' Mountain, O'Leary Peak, 

 Sitgreaves Peak, and Mormon Mountain. In addition to these 

 cones the author shows that in the second period there were also 

 intrusions of peculiar laccoliths, which were not only intrusive, 

 but also partly extrusive in nature. 



The succeeding portion of the work is devoted to presenting 

 the results of the petrographic study of the lavas and of the gen- 

 eral petrology of the district. The different kinds of rocks, 

 rhyolites, dacites, latites, andesites, and basalts, have been thor- 

 oughly investigated, and their description is accompanied by 

 numerous chemical analyses made by the author. Under the 

 heading of the petrology there is given a series of statistical 

 studies of the results afforded by the chemical analyses, affecting 

 the origin and differentiation of the lavas, from various points of 

 view. The author here comes to some very interesting conclu- 

 sions which have a bearing on the general petrology of igneous 

 rocks, but it would be beyond the scope of this review to present 

 and discuss them. The whole work forms an important contri- 

 bution not only to our knowledge of the geology of this part of 

 the country, but also to the subject of theoretic petrology in gen- 

 eral, and the final conclusions which Dr. Robinson has drawn 

 from the results of his studies should be read by all penologists. 



l. v. p. 



