The Commutator Subgroup of a Group Generated by Two Operators.

in the San Gabriel and the San Gorgonio mountains about 40 and 90 miles to the east. But when it comes to matching the Glacial epochs of the coast with those of the Sierra Nevada as lately studied by Blackwelder,3 or of the northeastern United States, difficulty is encountered. A simple solution of the difficulty would be to equate the Budu coastal epoch with the next-to-last Glacial epoch, the Tahoe stage of the Sierra Nevada; and the Memo coastal epoch with the last Glacial epoch, the Tioga stage of the Sierra; but that stage seems too brief for the accumulation of the heavy Memo cover, and too recent to be followed by the great abrasional excavation of Santa Monica Bay by the advancing Monic waves. If escape from this embarrassment is sought by shifting the Budu and the Memo intervals back to the Sherwin and Tahoe stages of the Sierra Nevada, then the coastal record of the late Monic episode of sea withdrawal and return seems too brief to correspond even to the short-lived Tioga stage. The glacial chronology of the coast is therefore left uncertain for the present. [A fuller discussion of the problem here outlined will shortly appear in the Bulletin of the Geological Society of America.] I W. H. Hoots, "Geology of the Santa Monica Mountains, California," U. S. Geol. Surv., Prof. Paper 165 C (1930). 2 R. A. Daly, "Swinging Sea Level of the Ice Age," Bull. Geol. Soc. Amer., 40, 721-734 (1929). I Eliot Blackwelder, "Pleistocene Glaciation of the Sierra Nevada," Bull. Geol. Soc. Amer., 40, 865-922 (1931).