130 T. Mellard Reade — Exfoliation of Gneiss in Brazil. 



in mountain torrents where crystalline rocks abound, where some 

 of the stones are completely rounded and others are blunted and 

 subangular. 



But we are asked, how can you account for their portage except 

 b}^ ice ? What other force was capable of bringing them to where 

 they are found from places so distant as Norway and Denmark ? 

 I reply that whatever force brought them it could not possibly have 

 been ice, for several very potent reasons, to which we will now turn. 

 The first reason is, that the foreign stones do not all come from the 

 same direction ; and as they are found in the same beds mixed 

 together, it seems impossible to attribute them to the portage of ice, 

 which moves along perfectly defined fixed lines of least resistance. 

 ]\Ir. Goodchild, it is true, fathered some years ago an extraordinary 

 theory according to which there could be currents flowing athwart 

 each other in a great mass of almost solid matter like ice. I do not 

 know whether he still holds this view or not. To me it was, and it 

 remains, incomprehensible. It is quite true that when a glacier is 

 coming down a wide valley with a slight momentum, and is joined 

 by a subsidiary glacier from an influent valley (which may be 

 steeper), the intruding glacier, whose flow is more rapid, will 

 thrust the other glacier back for a short space, and it will appear as 

 if the ice were moving in two directions, and this will continue so 

 till the two ice-streams have coalesced. But this, which is a 

 perfectly simple and credible mechanical process, and only involves 

 the adjustment of two rivers of ice with initially varying force and 

 direction when accommodating themselves to a common path, is a 

 very different matter to divergent currents arising or existing in 

 ice-sheets and being continued in them when the ice is supposed to 

 be moving aci'oss natural obstacles. Such a process, and the 

 thrusting back of one glacier by another, is very different to currents 

 of ice crossing each other in a great ice-mass. Such a process is 

 to me incredible, nor have I been able to find any physicist to 

 credit it. If we cannot appeal to divergent currents in the ice, how 

 are we to explain the bringing together of stones from contrary 

 directions to the same place by means of ice ? 



[To be concluded in our next Number.) 



VII. — The Exfoliation of Gneiss in Brazil. 

 By T. Mellard Eeade, C.E., F.G.S., F.R.I.B.A. 



PROFESSOR J. C. BRANNER has written a most interesting and 

 valuable treatise on the " Decomposition of Rocks in Brazil," 

 of which he has kindly sent me a copy.^ All who want to study 

 agencies of decomposition and denudation as they act in tropical 

 countries cannot do better than read Professor Branner's clear and 

 well-digested exposition, illustrated as it is by excellent reproductions 

 of photographs and by figures and diagrams. 



Among the many remarkable effects brought about by changes 



' "Decomposition of Eocks in Brazil" : Bulletin Geo. Soc. America, vol. yii, 

 pp. 255-314, pis. x-xiv. 



