VOLUME XXV 



THE 



NUMBER 2 



Journal of Geology 



A Semi-Quarterly 



Edited By 



THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY ' 



With the Active Collaboration of 

 SAMUEL W. WILLISTON, Vertebrate Paleontology ALBERT JOHANNSEN, Petrology 



STUART WELLER, Invertebrate Paleontology ROLLIN T. CHAMBERLIN, Dynamic Geology 



ALBERT D. BROKAW, Economic Geology 



ASSOCIATE EDITORS 



SIR ARCHIBALD GEIKIE, Great Britain 



CHARLES BARROIS, France 



ALBRECHT PENCK, Germany 



HANS REUSCH, Norway 



GERARD DeGEER, Sweden 



T. W. EDGEWORTH DAVID, Australia 



BAILEY WILLIS, Leland Stanford Junior University 



GROVE K. GILBERT, Washington, D.C. 



CHARLES D. WALCOTT, Smithsonian Institution 



HENRY S. WILLIAMS, Cornell University 



JOSEPH P.IDDINGS, Washington, D.C. 

 JOHN C. BRANNER, Leland Stanford Junior University 

 RICHARD A. F. PENROSE, Jr., Philadelphia, Pa. 

 WILLIAM B. CLARK, Johns Hopkins University 

 WILLIAM H. HOBBS, University of Michigan 

 FRANK D. ADAMS, McGill University 

 CHARLES K. LEITH, University of Wisconsin 

 WALLACE W. ATWOOD, Harvard University 

 WILLIAM H. EMMONS, University of Minnesota 

 ARTHUR L. DAY, Carnegie Institution 



FEBRUARY-MARCH 1917 



ON THE HYPOTHESIS OF ISOSTASY - W. D. MacMillan 105 



THE MIDDLE PALEOZOIC STRATIGRAPHY OF THE CENTRAL ROCKY MOUNTAIN 



REGION. I - C. W. Tomlinson 112 



s 



SOME FACTORS AFFECTING THE DEVELOPMENT OF MUD-CRACKS E. M. Kindle 135 



DOWNWARPING xVLONG JOINT PLANES AT THE CLOSE OF THE NIAGARAN AND 



ACADIAN -_--_.._-_- Lancaster D. Burling 145 



THE WESTERN INTERIOR GEOSYNCLINE AND ITS BEARING ON THE ORIGIN 



AND DISTRIBUTION OF THE COAL MEASURES - - Francis M. Van Ttjyl 150 



A DECIMAL GROUPING OF THE PLAGIOCLASES - - - - - F. C. Calkins 157 



STUDIES FOR STUDENTS: A CLASSIFICATION OF BRECCIAS - W. H. Norton 160 



REVIEWS 195 



THE UNIVERSITY OF CHICAGO PRESS 



CHICAGO, ILLINOIS, U.S.A. 



Agents 



THE CAMBRIDGE UNIVERSITY PRESS London and Edinburgh 



KARL W. HIERSEMANN, Leipzig 



THE MARUZEN-KABUSHIKI KAISHA, Tokyo, Osaka, Kyoto, Fukuoka, Senda 



THE MISSION BOOK COMPANY, Shanghai 



