ABSTRACTS OF PAPEES 31 



Prof. B. Willis : The mechanics of overthrusting and underthrusting are 

 determined by local conditions, among which the relative vertical positions of 

 active pressure and passive resistance are most influential. If the line of 

 pressure be dii*ected above the resistance, we have overthrust. If the pressure 

 be directed under the resistance, we have underthrust. In general, I dissent 

 from the author's conclusions as to the relative position of pressure and re- 

 sistance in the cases cited by him. What he calls pressure I would call resist- 

 ance, and vice versa. 



Dr. Stephen Tabek : Professor Hobbs has referred to the destruction of 

 bridges during earthquakes because of the approach of their abutments as 

 proof of the crustal shortening of the earth. I have examined quite a number 

 of bridges damaged and destroyed by earthquakes. In all cases when there 

 was an approach of the abutments, it could be explained as the result of the 

 slumping of superficial material on which the abutments had been built. Abut- 

 ments built on solid rock are not permanently displaced toward one another, 

 although the bridge trusses sometimes hammer against the stones in the abut- 

 ments until the rock is pulverized. Evidence of slumping is often furnished 

 by the formation, farther back, of cracks running parallel to the streams. 



Prof. W. H. HoBBS : I have elsewhere shown how the drawing together of 

 the piers of bridges is usually on loose soil and explained it by a localization 

 of the slipping of the loose formations over the hard rocks. The Assuring 

 along river banks is a practically uniform observation in regions affected by 

 earthquakes, but it does not indicate tension. It is purely a result of shocks 

 which throw off layers of earth of the banks, just as the last of a series of 

 boys in line is thrown off when the line is pushed. 



SOME POINTS IN THE MECHANICS OF ARCUATE AND LOBATE MOUNTAIN 



STRUCTURES 



BY FRANK BURSLEY TAYLOR 



{Abstract) 



Following the suggestion of Stiess, a comparison is made of the patterns of 

 marginal moraines of continental ice-sheets and the marginal chains of Ter- 

 tiary fold-mountains marking the southern periphery of the Eurasian conti- 

 nental crust sheet. Certain similarities of arcuate and lobate forms seen in 

 the marginal structures of both ice and crustal sheets are very striking, and 

 it is pointed out that, in spite of the wide differences in the general character- 

 istics, composition, and physical properties of ice and rock, these closely sim- 

 ilar marginal forms and their relations to the sheets of which they form 

 marginal parts prove that both ice and rock sheets move in conformity to the 

 same law, and that they behave alike when they find unobstructed channels 

 of easy flow. The arcs and lobes of the Tertiary peripheral mountain ranges 

 of Eurasia, especially in the interval between the Philippines and Gibraltar, 

 exhibit a striking uniformity of strength and intensity or development along 

 the whole interval, and in genei'al, wherever there is exceptional development, 

 either in greater or less strength than the average, there is an evident reason 

 for it. This means general southward crustal movement toward the folded 

 margin. 



