LOCATION OF AREA

The San Franciscan volcanic 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, as shown by the shaded space on the index map forming figure 1. The center of the field lies about 50 miles south of the Grand Canyon of the Colorado and the southern boundary is in part coterminous with that of the San Francisco Plateau, which forms the southwestern division of the great Colorado Plateau.

The region is easily reached, for the main line of the Atchison, Topeka, & Santa Fe Railway traverses it from east to west for more than 60 miles. Flagstaff, a town of 1,500 inhabitants 10 miles south of the summit of San Francisco Mountain, is on the railroad, amid a branch line runs from Williams, 34 miles farther west, to the Grand Canyon. All the more important points of interest in the field may be reached without difficulty by wagon, and outfits may be obtained at Flagstaff.

OUTLINE OF THE REPORT

This report deals primarily with the volcanic phenomena of the region as determined in the field and laboratory. Chapter I contains a brief description of the geography of the field and Chapter II is devoted largely to the sedimentary formations and structure. The rest of the report Chapters III to VI—treats entirely of the various features of the volcanoes and igneous rocks, both individually and collectively. Detailed descriptions of the volcanoes and lava fields are given in Chapter III; the volcanic history of the region and its correlation with the general history of the surrounding country are presented in Chapter IV. These two chapters will presumably suffice for the general reader who may desire to become acquainted with the broader volcanic features of the region. Chapter V (Petrography) is devoted entirely to the detailed description of the individual igneous rocks of the region, as represented by a selected set of type specimens. In Chapter VI (Petrology) is presented a discussion of the igneous rocks considered collectively—that is, as a series of genetically related members. These last two chapters will be more especially interesting to petrologists, although there is considerable matter in the last chapter which may also be of interest to the general reader.

EXTENT OF FIELD WORK

The field work on which the report is based was carried on during the summers of 1901 to 1903, a portion of the time, however, being occupied by side trips to the Grand Canyon of the Colorado, the Verde Valley, and the Moqui Buttes. It was the original intention to study only San Francisco Mountain, but scattered observations made during the first summer at other localities, especially at Elden Mountain and Kendrick Peak, seemed to indicate that the region would repay wider study. The work was accordingly extended so as to embrace all the large cones that lie in the vicinity of San Francisco Mountain and some 2,000 square miles of the surrounding plateau country. The more detailed work was confined to the large cones and the laccoliths, as they presented the greatest variety of phenomena within the smallest space. Reconnaissance work was carried on in the surrounding country more especially for the purpose of determining the limits of the widespread basalt flows, their relation to the underlying sedimentary formations, and the character of those formations.