560 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [June 20, 



especially if its motion were slow ; and the effect of friction would be 

 to place the pebble on end. It will be easily seen in the figure that 

 the friction throughout the portion a will tend to place the axis 

 horizontal, while that throughout h will tend to place it vertical. It 

 is evident that the latter will preponderate. 



I doubt, however, whether in nature we have cases of pebbles 

 which have been arrested while descending by their own gravity. 

 The action of frost, as indicated already, seems to be a more probable 

 cause of their descent ; and their verticahty may be thus accounted 

 for. The upward motion of the matrix, while expanding under the 

 influence of frost, will be, relatively to the pebble, gradually increa- 

 sing from its lowest point upwards. The friction will therefore be 

 greater towards the upper end of the pebble, and the effect will be 

 similar to that just described, but more considerable. 



It will be observed that the pebbles near the sides of a furrow of 

 trail are more generally vertical than those in its more central parts. 

 This has arisen from the dragging of the materials from above down- 

 wards against the wall of the cavity, while it was being widened 

 and deepened. If a line be drawn through the centre of the pebble 

 parallel to the section of the side of the cavity, the motion of 

 the particles of the matrix between this line and the wall will, from 

 the retarding effect of friction against the wall, be less than on the 

 further side. Now, since the pebble tends to partake in the motion 

 of the enveloping matrix, it is clear that the effect produced wiU be 

 a tendency to roll it over, in the same direction that a pebble would 

 roU down the wall of the furrow. This tendency to roll, however, will 

 be resisted by the imperfect mobility of the particles of the matrix 

 inter se ; and the resistance will be the greater the more the figure 

 of the pebble deviates from sphericity. It seems, then, that when the 

 pebble has assumed a position in which the moment of the rolling 

 force is so far reduced by its position as to be counterbalanced by 

 this resistance, it wiU remain permanently so, and move forward 

 Avithout further rolling. The position in question will be one in 

 which the longer axis is approximately parallel to the wall of the 

 cavity. And hence the pebbles at the sides wiU be more or less 

 nearly vertical. 



Though I cannot give any measurements, I may observe that the 

 effects of frost seem to have extended at the time the warp was 

 formed much deeper than they do at present. 



Mr. Trimmer has noticed, in his paper " On the Soils of Norfolk " 

 (Journ. Eoyal Agricultural Society, vol. vii. pt. 2), that, as a rule, 

 the warp is thicker upon the tablelands, thinner on the flanks of 

 hills, and, again, thicker in the bottoms of valleys. In short, there 

 has been a slow drifting action from the higher to the lower 

 grounds ; and it will accordingly be observed that the warp on the 

 sides of hills and at their feet tallies less "with the fundamental rock, 

 but consists chiefly of the materials brought from the higher grounds. 

 How, then, does this drifting take place ? Unless I am mistaken, it 

 is partly subterranean. Every one must have noticed the singularly 

 eroded line of junction often existing between the warp and the 



