1866.] 



FISHER WAKP. 



559 



had commenced, roots have played an important part in the work, 

 and the earthworm, as Darwin first pointed out *, has contributed a 

 by no means inconsiderable share to the general result. 



Of these agents I will touch only upon the work of frost and rain. 

 And first of frost. Suppose a mass of loam containing a pebble to be 

 frozen, what takes place ? The loam, by virtue of the water which it 

 contains, expands, and this expansion necessarily takes place entirely 

 upwards. The pebble does not expand. The consequence must be 

 that some of the loam, which surrounded the pebble laterally, will be 

 pressed over its upper surface. When the thaw occurs the soil sinks 

 again, and, there being now more material above the pebble than be- 

 fore, it is forced to imbed itself at a lower level. There is another 

 action which conduces to the same result. The frost enlarges in 

 every direction the hole in which the pebble lies. As soon as the 

 ice melts, the pebble sinks to the bottom of the hole, by this means 

 gaining a depression equal to the thickness of the coating of ice 

 which surrounded it. The subsequent rain and the subsidence of the 

 soil fill up the vacuity. 



It is evident that from this cause the larger stones will descend 

 faster, because their relative displacement in the general loam will 

 be in proportion to their linear dimensions ; and the materials of the 

 warp will be arranged vertically according to the relative magni- 

 tude of the constituents, to whatever depth the frost may penetrate : 

 and we may expect to find a layer of stones in its lower part, as in 

 fact we do. 



This action will, however, be modified by the eiFects of rain. For 

 that, on the other hand, carries down the finer particles of the soil 

 through the interstices among the coarser materials, and fills the 

 spaces which the frost opens ; and the examination of the warp be- 

 neath a lens reveals this arrangement of the finest clay enveloping the 

 sandy particles. Pebbles taken out of the warp are also covered 

 with a fine polished coating of clay. Hence any layer of clay, 

 however thin, originally on the surface will be distributed equably 

 throughout the whole depth of the warp. 



The erect position, already noticed, 

 of the included pebbles, is in many cases 

 common to the warp as well as to the 

 trail. This effect is probably not always 

 produced in the same way. The fol- 

 lowing appear to be some of the possible 

 causes of the phenomenon. 



It is well known that a plate, or 

 other heavy lamina, will sink through 

 water with its plane horizontal. In the 

 same manner a pebble of an elongated 

 elliptical form would sink through water 

 with its axis horizontal. But if it sank 

 through mud the friction would play a 

 more important part than the resistance, 

 * Transactions Geol. See. 2nd ser. vol. v. p. 505. 



Fig. 7. — Diagram illustra- 

 ting the motion of a pebble 

 in a plastic medium. 



