2L8 



SCIENCE. 



[N. S. Vol. XXI. No. 528. 



Eldorado County southward to the national 

 boundary. 



According to Dr. G. P. Merrill, of the 

 National JMuseum, the so-called asbestos 

 (fibrous serpentine) of the Thetford Mines, 

 Canada, and elsewhere, fills cavities which 

 were made by the shrinkage of the massive 

 serpentine in which the fibrous material 

 occurs, and he advances arguments to prove 

 that the filling process is due to crystalliza- 

 tion from the walls of the cavities inward. 



Messrs. Ralph Arnold and A. M. Strong, 

 of the California State University, de- 

 scribed at length the crystalline rocks of 

 the San Gabriel Mountains near Pasadena, 

 Cal. The last of the petrographical papers 

 was by Dr. G. M. Murgoci, of Bucharest, 

 Roi;mania, and concerned the origin of the 

 peculiar rock known as riebeckite granite, 

 suggesting that the change from normal 

 granite was due to heavy pressure com- 

 bined with motion. 



Five papers on physiographical geology 

 were presented, three of which were read 

 in full. Professor N. M. Fenneman, of 

 the University of Wisconsin, in a paper 

 on the control of the form of contact sur- 

 faces by marine denudation, laid down 

 the principles that the nature of a surface 

 of unconformable contact between strata 

 is determined by two factors: (1) the 

 topography of the early land surface, and 

 (2) elifif erosion during submergence. The 

 first element would ])reserve the former 

 land-surfaces in the subsequent beds, while 

 the dominance of the second element would 

 make the contact surface in every case a 

 phine. 



Professor R. S. Tarr, of Cornell Uni- 

 versity, described some drainage features 

 of southern central New York showing the 

 relation of the pre-glacial valleys to the 

 present surface. In many instances along 

 the divide between the Susquehanna and 

 St. Lawrence drainage system there is a 

 condition of lowered divides, across some of 



which, as in the Tioughnioga valley, east 

 of Cortland, and Cayuta Creek valley, west 

 and south of Van Ettcn, the present drain- 

 age passes. Three theories may be ad- 

 duced to account for these phenomena : 

 glacial erosion, erosion by ice-fed stream 

 and head-water erosion during rejuvena- 

 tion. Evidence from valley form, glacial 

 deposits and hanging tributary valleys is 

 presented to prove that these drainage fea- 

 tures are in many cases, if not in all, due 

 to changes of earlier date than the advance 

 of the AViseonsin ice sheet. While the 

 influence of possible earlier ice advances, 

 of which no evidence has been found in 

 this region, is not eliminated, the facts so 

 far discovered favor the hypothesis of re- 

 juvenation rather than of glacial action 

 during earlier ice advance. 



The next paper pertained to hanging 

 valleys and was by Professor Israel C. 

 Russell, of the University of Michigan. 

 He recognizes four classes of such valleys, 

 each of which contains several varieties. 

 The author considers that too much stress 

 has been laid upon the existence of lateral 

 glaciated hanging valleys on the sides of 

 glaciated troughs and he advances evidence 

 to show that in certain instances at least 

 such valleys are not due in a conspicuous 

 manner to differentiation of glacial erosion. 

 The study of glaciated hanging valleys is 

 intimately connected with a still greater 

 problem, namely, the origin of the leading 

 features in the relief of such mountains as 

 the Sierra Nevada range and the Cascades. 

 There is good reason for thinking that 

 these two ranges were deeply stream-sciilp- 

 tured prior to the glacial epoch. 



Under the head of physical and striic- 

 tural geology twelve papers were read. 

 Professor C. K. Leith, of the University 

 of Wisconsin, discussed in masterly fashion 

 the present state of knowledge of the siib- 

 ject of rock cleavage, with special reference 

 to recent publications by Dr. Becker and 



