78 NATURAL SCIENCE. February, 



accepted from faith in their work. It was clear from remarks made 

 at the conclusion of Professor Bonney's lecture that some English 

 geologists have not appreciated how utterly the theory of the Mesozoic 

 age of the crystalline schists of the St. Gothard area has been 

 demolished by Professor Bonney's paper of 1890. To that paper no 

 serious reply has ever been attempted. In fact, from the long list of 

 literature on the St. Gothard area, published in the " Livret Guide " 

 prepared by the Swiss geologists for the Geological Congress of 

 1894, th^t paper was carefully excluded. The failure of the Swiss 

 geologists to accept the challenge there made to them to substantiate 

 the accuracy of their observations, has relieved geology from an 

 incubus of error, for release from which Professsor Bonney deserves 

 the thanks of all geologists. 



SCHISTOSITY AND SlATY ClEAVAGE. 



Mr. G. F. Becker's paper on this intricate subject in a recent 

 number of the journal of Geology (vol. iv., pp. 429-448) is a lucid non- 

 technical presentation of his theory of these rock-structures, as more 

 fully elaborated in an earlier memoir. Of the different kinds of 

 deformation to which rock-masses may be subjected, the author con- 

 siders specially two distinct types of strain, which he names " pure 

 shear " and " scission," though in general the effects of the two must 

 be associated. In the former the directions of the planes along which 

 flowing movement takes place are continually changing ; in the latter 

 one of the two directions of flowing is fixed. The author further con- 

 siders the effect of viscosity as superposed on that of elastic resistance 

 and as modifying the results. He submits that the resistance of the 

 rock to rupture must necessarily be modified along those planes which 

 have, during the process of deformation, been planes of flow ; and he 

 decides on experimental grounds that they will be planes of weakness 

 rather than of increased strength, or, in other words, they will be 

 planes of schistosity or cleavage. 



On this hypothesis, pure shear would give rise to structures 

 parallel not to a definite plane, but to planes lying between two limit- 

 ing planes, making a certain angle with one another which would 

 increase with the amount of shear ; and there would be two sets of 

 such planes, equally inclined to the direction of maximum compres- 

 sion. In the other ideal case, that of pure scission, one set of planes 

 would be strictly parallel, while the other set would range through an 

 angle twice as great as in the former case for a like amount of deforma- 

 tion. The author considers, however, that, owing to viscosity, the 

 latter set would usually not become effective. The argument here is 

 not convincing, for if viscous resistance be proportional to the rate of 

 shearing (as it is in a homogeneous substance), and if this rate be 

 extremely slow (as is usually presumed), this part of the resistance 

 may be a vanishing quantity at every point of time. 



