1893.] evidence for the Recurrence of Ice Ages. 107 



paste will scratch and furrow the larger softer pebbles along 

 which they are driven, see Nos. 52, 53, 54, 55, 56. Small tough 

 pebbles will often deeply indent softer pebbles without breaking- 

 them or being broken by them. For the permanent deformation of 

 rock without fracture, however, time is an element which has to 

 be taken into account, but for our present purpose we need not 

 further consider this point. 



In the case of crushed conglomerates we often find that the 

 pebbles have been broken and the several parts have been separated 

 somewhat or shifted and recemented by mineral matter, see Nos. 

 57, 58, 59, 60. This I have never known to be the case in glacial 

 deposits. Sometimes the conglomerate has been crushed in such 

 a manner as to cause not only a protrusion of the pebbles through 

 the matrix but also a number of small faults throughout the mass 

 on the face of which it will be seen that the strias produced by 

 slickensiding run across the matrix and included pebbles alike, a^s 

 seen on Nos. 50, 51. Striated floors have been noticed in glacial 

 drift, but they were all due to more or less horizontal movements, 

 whereas the striated faces in this conglomerate are at all angles to 

 the planes of the bedding. 



When however a thrust happens to have taken place between 

 the conglomerate and underlying rock we have of course a 

 polished, striated and grooved floor just where, on the supposition 

 that the conglomerate is of glacial origin, we should look for the 

 glaciated surface on which the ancient boulder drift was de- 

 posited. One very striking example of this occurs in the same 

 area as that in which so many striated blocks have been procured 

 from the conglomerate. It might seem that there was cumulative 

 evidence for the glacial origin of the deposit. Scratched stones in 

 a boulder drift resting upon a striated surface of solid rock ! But 

 when we examine the evidence more carefully it all breaks down. 

 This striated surface occurs along a thrust plane which can be 

 clearly made out on the ground. And even were it not so the 

 condition of the surface itself is sufficient to show that it is due to 

 earth movement and not to glaciation. The rock itself can be 

 seen on the specimens Nos. 61 and 62 to be squeezed out in the 

 direction of the movement, and a thin layer of mylonite covers 

 the whole. Now the evidence is turned against the glacialists, 

 and it may be urged that where there were such rock crushings as 

 those which are known to have taken place in Holbeck Gill it 

 would be curious if here and there we did not find that there had 

 been a differential movement between masses of such unequal 

 structure as the conglomerate and the solid grauwacke on which 

 it lay. 



The character which is most constant in pebbles which have 

 been subjected to this crushing action in a conglomerate and at 



