TITLES AND ABSTRACTS OF PAPERS 31 



type. Contrary to the rather generally accepted view, it was argued that the 

 distribution of the active tangential thrusts during the formation of the arc 

 is centripetal and not centrifugal with respect to the center of the arc. This 

 requires that in the formation of anticlines the active thrust which produces 

 them comes from below and in front and not from above and behind the rising 

 arch. For this reason folds in later stages become underturned and under- 

 thrust, but are never "overturned" or "overthrust." True, overturning and 

 overthrusting take place, however, in the snouts of glaciers, where overturned 

 and overthrust folds are produced, though with different characters dependent 

 on this manner of formation. 



A study has been made of the actual loads and of the percentage of total 

 loads which a competent stratum may lift from underlying formations while 

 rising to form an arch, and deductions have been drawn concerning the effect 

 on the form of the fold, etcetera. The special conditions which determine 

 failure (sliding) within an arch of strata, and in later stages the complex 

 Deckenhau of the Alps with its drag-folds and listric surfaces at the forward 

 end, also received consideration. 



Eead in full from manuscript. 







EARLY TERTIARY GLAGIATION IN THE SAN JUAN REGION OF COLORADO 

 BY WALLACE WALTER ATWOOD 



(Abstract) 



In conducting an areal survey of the northwestern quarter of the Montrose 

 Quadrangle, which is located near the northwestern base of the San Juan 

 Mountains, a number of exposures were discovered in which there are glacial 

 deposits overlain by conglomerates and volcanic rocks of known Tertiary age. 

 These glacial deposits are divisible into two distinct sheets of till. The ice 

 was presumably in the form of Alpine glaciers. 



Presented in full without notes. 



Discussion 



Dr. I. C. White: Mr. Atwood might have added Brazil to the list of coun- 

 tries where Permian glaciation had been discovered. The first definite and 

 positive announcement of Brazilian glaciation was made in 1906 by myself 

 and the evidence therefor published in my final report of the Brazilian Coal 

 Commission, issued in 1908. Doctor Derby had previously intimated that 

 Permian glaciation in Brazil was possible, but had made no positive statement 

 on the subject. Doctor Woodworth, of Harvard University, confirmed my 

 conclusions by finding striated boulders in the Permian tillite of Brazil in 1910. 



Dr. R. D. Salisbury : In connection with the closing Paleozoic glaciation, it 

 is now known that glacial beds of Permo-Carboniferous age occur in the Sierre 

 Ventane Hills of Argentina. This discovery was made by Doctor Keidel. 



Professor Salisbury asked Professor Coleman whether it was now possible 

 to state the age of the Canadian glacial beds more accurately than heretofore. 



Professor Coleman replied that the beds were at the base of the Lower 



