Geology and Mineralogy. 407 



significance. Its publication will place the subject of rock cleav- 

 age in a much more satisfactory shape and be of material assist- 

 ance to all structural geologists." The writer divides secondary 

 or induced cleavage into flow-cleavage and fracture-cleavage. The 

 first is considered as depending upon a parallel arrangement of 

 mineral particles and is shown to be developed by rock flowage, 

 the cleavage, as held by Van Hise and Hoskins, developing at any 

 instant normal to the greatest pressure, but the final direction of 

 cleavage may be inclined to the direction of greatest pressure 

 which has produced the deformation. Fracture-cleavage on the 

 other hand is considered as not dependent upon a parallel arrange- 

 ment of mineral constituents and as arising in a plane of shear in 

 the outer zones of the earth's crust. The writer follows the 

 inductive method of studying the facts in the field and labora- 

 tory and pointing out their significance. j. b. 



11. Experiments on Schistosity and Slaty Cleavage; by 

 George F. Becker. Bulletin 241, U. S. Geol. Surv., 1904, pp. 

 34, 7 pis. — This bulletin embraces the results of valuable experi- 

 ments upon clay and ceresin in a "scission engine" of the 

 author's invention in order to test disputed theories of cleavage. 

 The results show, according to Dr. Becker, that in these cases the 

 cleavage arose on planes of shear and not in the planes sub- 

 jected to greatest pressure. This is in conformit}^ with the 

 author's conclusions published ten years previously and these are 

 regarded by him as establishing his contention that all cleavage 

 is to be regarded as arising in planes of shear. As noted in the 

 previous review, Professor Leith grants the occurrence of this 

 form of cleavage, but regards it as only of partial application to 

 the natural phenomena and distinct from the more common flow- 

 cleavage. Leith also differs from Dr. Becker in the interpreta- 

 tion of the experiments, regarding the results as due to flow- 

 cleavage. J. B. 



12. Die Alpen im JEiszeitalter ; von Dr. Albrecht Penck, 

 Professor an der Universitat Wien und Dr. Eduard Bruckner, 

 Professor an der Universitat Bern. Part 6, pp. 545-656, 1904; 

 Part 7, pp. 657-784. Leipzig, 1905. — These two parts continue 

 the discussion of the Alps in the Glacial Period, and it is ex- 

 pected that the series will be completed upon the issuance of one 

 more part. The volumes are well illustrated by profiles, maps, 

 and photographs, and the names of the authors are a guarantee 

 as to the quality of the work. 



The subjects discussed in part 6 include the glacial history of 

 the Reuss, Aar and Rhone valleys, the physiographic features 

 and the nature of the interglacial plant remains. Part 7 deals 

 chiefly with the French and southern Alps and discusses the 

 paleolithic finds and their relation to the glacial and interglacial 

 times. This work is of the utmost interest to anthropologists as 

 well as to glacialists and physiographers and will rank for many 

 years as a standard contribution to the Quaternary history of 

 Central Europe. j. b. 



