784 MR. ^Y. BAELOW o:n" the horizontal 



The phenomenon of creep* may be defined as the thickening of 

 the parts of beds from which a load of superincumbent rock has been 

 lifted, caused by a thinning of the adjoining parts of the same beds, 

 which continue loaded, some of the substance of the latter being 

 squeezed out to furnish the material for the thickening. The effects 

 are, however, so extensively diffused that, although it is the hori- 

 zontal components of the motions of the rock-particles which alone 

 determine the extent of a creep, it is the vertical components of 

 these motions which alone force themselves on our attention ; thus 

 Buddie tells us, " he has never noticed any tendency to a sliding or 

 sideway movement in any subsidence of strata occasioned by the 

 working of the coal, except the slight obliquity occasioned by the 

 offbreak at the sides of the settlement where the strata are bent 

 down and cracks formed " f. 



If we regard the phenomenon recorded by Dutton as an instance 

 of creep on a large scale, we must conclude that a lateral extension 

 of the beds still remaining heavily loaded has taken place, and that 

 a large mass of material has, somewhere below the surface, been 

 squeezed out from beneath the cliffs. Further, the movement of 

 this large mass must have produced a considerable horizontal thrust, 

 which, as the process was no doubt very slow, and the lower ground 

 at the foot of the cliffs of considerable extent, would be transmitted 

 through the rocks underlying this lower ground to a very great 

 distance. 



The effects of such a horizontal thrust are, in the cases referred to, 

 and in most other cases where masses of rock are similarly bounded 

 by precipices or steep slopes, hidden from view, but there are in- 

 stances where they are to be traced. Thus many evident examples 

 of plication traceable to horizontal thrusts produced by gravitation 

 are to be seen in Glacial drift. 



And from the fact that comparatively small masses of rock have 

 been able by their weight to squeeze up plastic Boulder-clays and 

 soft sandy layers on which they rested into folds and contortions, we 

 may fairly conclude that the unequal distribution of weight at the 

 earth's surface, due to the presence of lofty cliffs and mountainous 

 blocks, has been able during long periods of time to produce consider- 

 able plication, even of the more intractable rocks, in the same way. 



It can scarcely be doubted that many instances of plication 

 generally attributed to secular contraction of the earth's crust are 

 traceable to the cause I have named. This wiU especially be the 

 case with subsidiary plications found on the flanks of mountains ; for 

 wherever there are great inequalities of surface, rocks far beneath 

 the surface and consequently having considerable plasticity, will be 

 materially affected by the unequal distribution of the weight of the 

 rocks above them, and will spread in the same way as the Boulder- 

 clays referred to have done. 



It has been suggested that much of the contortion and upheaval 

 of the later Tertiary rocks of the sub-Himalayan zone has been 



* See ' Student's Elements of Geology,' 2nd edition, p. 55. 

 t Proc. Geol. Soc. vol. iii. 1842, p. 149. 



