~ VOLUME XX = ‘ NUMBER 5 
THE 
JOURNAL orf GEOLOGY 
A SEMI-QUARTERLY 
_ EDITED BY 
THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY 
With the Active Collaboration of 
SAMUEL W. WILLISTON - ALBERT JOHANNSEN _ WILLIAM H. EMMONS 
Vertebrate Paleontology Petrology Economic Geology 
STUART WELLER WALLACE W. ATWOOD ROLLIN T. CHAMBERLIN 
- Invertebrate Paleontology Physiography ynamic Geology 
ASSOCIATE EDITORS 
‘SIR ARCHIBALD GEIKIE, Great Britain GROVE K. GILBERT, Washington, D.C. 
HEINRICH ROSENBUSCH, Germany CHARLES D. WALCOTT, Smithsonian Institution 
THEODOR N. TSCHERNYSCHEW, Russia HENRY S. WILLIAMS, Cornell University 
CHARLES BARROIS, France JOSEPH P.IDDINGS, Washington, D.C. 
ALBRECHT PENCK, Germany JOHN C. BRANNER, Leland Stanford Junior University 
HANS REUSCH, Norway . RICHARD A. F. PENROSE, Jr., Philadelphia, Pa. 
GERARD DEGEER, Sweden WILLIAM B. CLARK, Johns Hopkins University 
‘ORVILLE A. DERBY, Brazil WILLIAM H. HOBBS, University of Michigan 
T. W. EDGEWORTH DAVID, Australia FRANK D. ADAMS, McGill University 
BAILEY WILLIS, Argentine Republic CHARLES K, LEITH, University of Wisconsin 
= . ve => JULY-AUGUST: 1912 
CONTENTS ree 
‘THE EVIDENCE OF THREE DISTINCT GLACIAL EPOCHS IN THE PLEISTOCENE 
HISTORY OF THE SAN JUAN MOUNTAINS, COLORADO 
Wattace W. ATWooD AND KirtLEY F. MATHER 385 
: THE OLD EROSION SURFACE IN IDAHO: A CRITICISM - Eiot BLACKWELDER 410 
GEACIATION OF THE ALASKA RANGE - ~- =~ |. Srepwen R. Capps. rg 
SOME SMALL NATURAL BRIDGES IN EASTERN WYOMING - _ V.H. Barner? 438 
ROCK-CUT SURFACES IN THE DESERT RANGES-  -  - -  - Swyey Patce 442 
SPECULATIONS REGARDING THE GENESIS OF THE DIAMOND. II 
j Orvitte A. DERBY 451 
_ THE ORDER OF CRYSTALLIZATION IN IGNEOUS ROCKS- -  - N.L. Bowen 457 
P PETROLOGICAL ABSTRACTS AND REVIEWS .- ¢- = -  - 0 he g65 
Che Anibersity of Chicago press 
CHICAGO, ILLINOIS 
- AGENTS: 
THE CAMBRIDGE UNIVERSITY PRESS, Lonpon anp EDINBURGH 
- WILLIAM WESLEY & SON, Lonpon 
TH. STAUFFER, LErpzic 
THE MARUZEN-KABUSHIKI-KAISHA, Toxyo, Osaxa, Kyoto 
