��THE SCIENTIFIC MONTHLY
��fine materal which would account for the irregular bedding in the ejecta in the encircling ridges.
Other explosion craters in south central Mexico have been described by Ordonez, notably the fine group of eleven in the Valle de Santiago* and the crater at Xico/ near Mexico. These craters are in a plain underlain by tuffs and the beds adjoining the holes are not disturbed. The craters are surrounded by ridges of ejecta and all are the products of a similar cause, but in some cases two or more stages of development are apparent The largest is more than a mile in diameter.
An explosion crater at Tac&mbaro, in Michoac4n, has been described by Bubio.* It contains a lake nearly a half mile in diameter, and the surrounding ridge of ejecta is more than 500 feet high on one side. The walls are basalt and the ejecta are ash, lapiUi and tuff, with rock fragments.
Hornaday^ and Lumholtz* have described a group of remarkable explosion craters in the northern part of Sonora, Mexico, fifteen to
���Fia. 6. Map of Past of Sonora Dksbbt in BCexico and Abieona, showing lo- cation of craters near Pinacate Mountains, after G. Sykes. P, Plnacate Mountains; H, Homaday Mountains; +, craters; 8, Sykes Crater; M, MacDougal Crater; B, Crater Elegante.
��twenty-five miles south of the International Boundary line, about 100 miles southeast of Yuma, Arizona. This area is shown in the follow- ing map :
4 * ' Les volcans du valle de Santiago, ' ' por E. OrdoiLeZi Soc. Antonio Alzate Mems., Vol. 14, pp. 299-326, pis. IV.-IX., 1897. Also **Les Crateres d 'explosion de Valle de Santiago," por E. Ordonez, Guide des Excursions Xe Cong. Geol. Int., Mexico, 1906, XIV., Excursion du Nord, 9 pages, plate.
CLos crateros de Xico," Bol. Soc. GeoU Mexicana, Tomo 1, pp. 19-24, 1906.
" El Axalapazco de Tadimbaro, ' ' por P. O. Bubio, Bol, Soc, Geol, Mexicana, Tomo "2, pp. 65-69, 1906.
7 **Camp Fires on Desert and Lava," by W. T. Hornadaj, Bcribner's, 1908.
8 New Trails in Mexico," hj Carl Lumholtz, Scribner's, 1912.