part 1] ANIS^IVERSAET ADDRESS OF THE PRESIDENT. Ixxxix 



ratio to the first, a ratio which depends on the nature of the 

 material and the character of the surface of separation. This 

 ratio is known as the ' coefficient of friction,' and is, numerically, 

 the same as the tangent of the angle of repose. For a flat-dressed 

 surface of stone the coefficient is about three-fifths of the weight 

 of the stone, for a surface such as that of a so-called 'thrust- 

 plane' it would not be less : consequently, to move a block of 

 rocks 5 miles wide would need a pressure equal to that due to the 

 weight of a column, of the same rocks and of the same cross- 

 section, having a height of at least 3 miles, or just about the limit 

 of height of column which average hard rock can bear without 

 crushing. 



From these figures it aj^pears that the maximum possible 

 Avidth of the overthrust must be somewhere about 5 miles, if it 

 moved as an inert mass under the influence of some external 

 impulse : for, if the width exceeded this limit, the stresses would 

 be greater than those which rock could bear or transmit, and relief 

 would be found in some other way than b}^ a general displacement 

 along the whole width of the overthrust ; but 5 miles is less 

 than half the width of the mass moved in the Highland over- 

 thrusts, it is not more than a tenth of that of the Scandinavian, 

 and a still smaller fraction of those which have been deduced 

 in the region of the Alps. From this it might seem to be 

 established that none of these overthrusts could possibly have 

 been produced, and that there must be some error in the obser- 

 vations, or the inferences which have been drawn from them as 

 to structure. 



This reasoning, however, is not justifiable. We have again a 

 case very like that which has been mentioned in connexion with 

 what are ordinarily understood as reversed faults, and once more 

 we have to face the alternative that the hypothesis of origin needs 

 correction, not the facts of observation ; but, before examining this, 

 it is necessary to refer to one possible means of getting over the 

 difficulty which has been encountered. If we might believe that 

 the coefficient of friction along the surface of the thrust Avas less 

 than that adopted in the calculation, the Avidth of the blocks 

 which could be moved would be correspondingly increased ; but not 

 in this way can sufficient increase be obtained, for even Avith the 

 most perfectly formed and lubricated surfaces in mechanism the 

 coefficient is not materially less than one-tenth, and the maximum 



VOL. LXXviT. y 



