March 28, 19(12.] 



SCIENCE. 



507 



if any did exist they would at present be sub- 

 merged. 



Pliocene reefs were extensively developed 

 along the Florida coast, for instance, along the 

 Caloosahatchee River. The genera of the 

 Pliocene corals are the same as those at present 

 living in the Floridian and Antillean seas, but 

 often there are appreciable specific differences 

 between the Pliocene and recent representa- 

 tives of the same genus. 



Mr. Bailey Willis spoke on the 'Conditions 

 of Overthrust in the Northern Rockies.' After 

 restating the facts relating to the overthrust 

 of Algonkian strata upon Cretaceous with a 

 displacement of more than seven miles, along 

 the eastern flanls of the northern Rocky Moun- 

 tains in northwestern Montana, Mr. Willis 

 presented a hypothesis of origin and develop- 

 ment of this structure. It is assumed that in 

 Cretaceous time Algonkian strata in this 

 region were essentially flat, and in consequence " 

 of subsidence were buried under Dakota and 

 Pierre sediments, with a shore line not far 

 from the position of the present mountain 

 range. Algonliian strata beneath the marine 

 area being- depressed and beneath the land^area 

 being raised, they were bent parallel to the 

 general trend of the shore. When later the 

 strata were compressed, the initial bend deter- 

 mined an anticline in this same position. Ero- 

 sion of the arch ciit deeply into Algonkian 

 beds and left the edges exposed and free to 

 move. Continued compression resulted in 

 their being thrust upward and northeastward 

 upon the eroded surface, until Algonlcian lime- 

 stones came to rest upon Cretaceous areas. 

 The structure closely resembles the Rome 

 fault, Georgia. The date of development is 

 inferred to have been early Tertiary. 



Mr. F. E. Matthes presented a paper on 

 'Glacial Erosion in the Northern Rockies.' 

 The range was shown to have been deeply dis- 

 sected before the advent of the glaciers. The 

 valleys were nearing maturity and had low 

 gradients; the glaciers which subsequently 

 occupied them had therefore but little fall 

 from their sources to their distal ends. They 

 moved slowly. and were of considerable thick- 

 ness. The lengths of the various trunk 



glaciers were small in proportion to the large 

 neve areas which they drained. 



The frequent occurrence of many valleys 

 radiating from one point was shovm on the 

 map. The effect of this arrangement upon 

 the valley glaciers was shown to have been a 

 general retardation of their flow and a conse- 

 quent increase in thickness above their j\mc- 

 tion. Some attained a thickness of over 3,000 

 feet in some parts of their course. 



The radiating system of ravines at the 

 heads of valleys was shown to be particularly 

 favorable to the development of cirques. At 

 least two sets of cirques at different elevations 

 are found in these mountains, indicating oscil- 

 lations of the neve line to as low as 6,000 feet 

 altitude. 



The definitions of the snow line as given in 

 three text-books now in use were compared and 

 found to be greatly at variance with each 

 other. A new definition was favored in which 

 the topographic element is given due weight, 

 and which makes the snow line virtually coin- 

 cident with the neve line as found on glaciers. 



The tendency of glaciers to flatten the grades 

 of their channels, beginning at the upper ends, 

 was shown to be productive of the step-like 

 profiles of glaciated valleys. The cause of 

 this tendency was sought in certain motions in 

 the interior of the glaciers, the explanation of 

 which was not attempted. 



The widening of the valleys by cliff recession 

 was emphasized as an important factor in pro- 

 ducing discordance between valleys. Discord- 

 ance was shown to be produced by (1) deepen- 

 ing of main valley, (2) widening of main 

 valley ; and to be diminished by (3) deepening 

 of side valley. Cases were pointed out on the 

 contour maps of valleys meeting with dis- 

 cordances ranging between 300 and 1,500 feet ; 

 also of several meeting with perfect accord- 

 ance. Nor were the discordances always in 

 inverse ratio to the drainage areas of the 

 respective valleys. 



The conclusion was reached that in no case 

 could the discordance of a side valley be taken 

 as a measure of the deepening of the main 

 valley. 



Alfred H. Brooks, 



Secretary. 



