1893.] evidence for the Recurrence of Ice Ages. 105 



to the nail boots and caudal corduroys of small Welshmen who 

 amuse themselves between school hours by sliding down the 

 smooth face of the rock. 



On the limb bone of a large Saurian, No. 37 and No. 38, from 

 the Kimeridge clay of Littleport, North of Ely, there are scratches 

 and grooves running in various directions. We have several other 

 examples of the same kind of thing. There is in the case of this 

 specimen enough to prevent our referring them to glaciation in 

 the character of the striae themselves. The short cross cuts for 

 instance which appear here and there and the abrupt termination 

 of the grooves on the face of the bone. But had only a fragment 

 of this specimen been preserved and that perhaps been somewhat 

 rolled, it would not have been so safe to pronounce that the 

 scratches could not have been of glacial origin. 



The cuts cannot have been produced at any later date than 

 the deposit in which the specimen was found as they are in places 

 covered by organisms which grew upon the bone as it lay at the 

 bottom of the Jurassic sea, and the fauna of the Kimeridge clay 

 precludes our entertaining the possibility of the occurrence of 

 glacial conditions in our part of the world during its deposition \ 

 Occasionally the same kind of thing may be seen on lumps of 

 phosphate of lime from the Cambridge Greensand as in No. 39 

 on which Plicatula sigillina has grown upon a striated surface. 



vii. Striations and polishing due to crushing of solid rock. 



When the faces of a fissured rock have been rubbed against 

 one another by earth movements under tremendous pressure, 

 either continuously in the same direction or with a to-and-fro 

 motion, as I have already pointed out, there is often a film of 

 some mineral formed over the surface, and I have already con- 

 sidered the cases in which the rock is homogeneous and a smooth 

 polished surface is produced. But when the surfaces in contact 

 are covered by irregularities such as minerals of unequal hardness 

 in the rock or in the fault the harder produce flutings and scratches 

 on the softer along the lines of movement. 



The immediate cause of the grooves may often be detected. 

 Sometimes they are due to such apparent trifles as worm tubes, 

 which, being filled with sandy material, are more solid and harder 

 than the rock that is thrust over them along the bedding planes 

 as seen on No. 40. Sometimes the lumps and grains which have 

 caused the fluting can be seen on the fault as in No. 41 which is 

 off the face of a landslip in the Barton Cliff, Hampshire, where the 

 accompaniments of faulting on a small scale may be studied. 



Sometimes movements have been repeated after a fault 



1 Vict. Inst., May 6, 1889. 



