XC PEOCEEDINGS OF THE GEOLOaiCAL SOCIETT. [vol. Ixxvil, 



width of block which could be moved would not be increased 

 beyond about 30 miles. The actual surface along which move- 

 ment took place being, to say the least, much less perfect than 

 those which give so small a coefficient of friction, the maximum 

 width that could be moved would in an}'- case be less than has, in 

 some instances, been shown by observation in the field. Resistance 

 to movement might, however, be reduced if the downward pressure 

 due to the weight of the upper block were, in some way or other, 

 temporarily relieved, and if this relief were complete there would 

 be no limit to the width of block that could be moved. It is 

 not easy to conceive the means by which this could be brought 

 about, nor is it necessary to consider the possibilit}^, for the 

 existence of mylonites, and other indications, of resistance to 

 movement, given by the deformation and fracture of rock, are 

 •eloquent of the resistances which had to be overcome when the 

 existing displacements were brought about. Taking these into 

 consideration, it is evident that the frictional resistance must have 

 been at least as great as is represented by the coefficient made use 

 of, so that the width of 5 miles must be regarded as a maximum 

 rather than a minimum limit of the width of the overthrust 

 which could be moved by pressure from without. 



From this we are led to the conclusion that the thrusts did not 

 move simultaneously over the whole of their extent, but partiall}^ 

 first in one part then in another, each separate movement involving 

 an area limited by the strength of the rocks and their power 

 to transmit, or resist the effect of, pressure. Some j^ears ago 

 it might have been said that any supposition of this kind was 

 physically impossible ; but at the present day the change of 

 volume which results from an alteration of the molecular group- 

 ing of the same chemical elements, expressed geologically as a 

 different mineralogical constitution of rocks having the same 

 chemical composition, or more brief!}'- as a change of mode of the 

 same norm, has opened up at least one means by which the desh'ed 

 effect might be produced. Doubtless the advance of knowledge 

 will open up other possibilities, some of which might be indicated, 

 though I shall not refer to them, as my present purpose is not 

 to deal with things themselves, but with the words in which they 

 are expressed. 



One result of the acceptance of any such process as has been 

 suggested is that the origin of overthrusts ceases to have any 



