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AN UNEECOGNISED AGENT IN THE DEFOEMATION 



OF EOCKS. 



By Eenest H. L. Schwakz, A.E.C.S., F.G.S. 



(P iihlished witJi the permission of the Geological Commission.) 

 (Eead February 4, 1903.) 



(Plates IV.-VI.) 



By the terra Deformation as applied to rocks there is meant 

 the shattering or crampling which may be seen in the folded 

 mountain ranges of all countries. 



In this paper the two phenomena of fracture and fiowage will not 

 be separated, for the difference is one of degree only. Any of the 

 forces that produce shattering or granulation will also produce 

 distortion without fracture under certain circumstances. 



The popular idea is that such deformation is due to some violent 

 upheaval; for it seems natural on seeing any great manifestation of 

 force to imagine that this has taken place with sudden and explosive 

 energy. This idea held the field for a very long time in the infancy 

 of Geology. But by the slow accumulation of facts and the patient 

 observation of Nature, it became more and more certain that the 

 great disturbances in the earth's crust took place with exceeding 

 slowness. 



It will be the object of this paper to show that this element of 

 time has not been sufficiently considered in the theories propounded 

 to explain the phenomena of rock-deformation. I will take certain 

 pebbles that have been subjected to great pressure in the laboratory 

 and in Nature, and will show that Nature w^orking with small forces, 

 but these acting for long periods, can deform the pebbles to a far 

 greater extent than we can working with much larger forces but 

 limited as to time. Although it will seem at first sight preposterous 

 to explain the forces at work in producing mountains from the effects 

 that can be produced on small pebbles, yet when it is remembered 

 that the ultimate elements of rocks are grains much smaller than the 

 pebbles used, it will be seen that we can only gain an insight into the 



