304 Proceedings of the Royal Physical Society. 



or broken ice, that the water, unable to bear it away, had 

 no choice but to break over the top, sweeping the shores 

 with levelling violence. It is thus in the ?io^-terraced por- 

 tion of the valleys of Northern England that we seem to 

 find traces of the most violent floods ; and there is a very 

 general concurrence of testimony that at these high levels 

 the river-gravels are thickest, coarsest, and most tumultuous, 

 besides being bouldered with blocks probably ice-dropped.* 



To facts of this kind we look as the proper evidence of 

 ancient floods. Their deposits must be proportioned to their 

 magnitude. We have a right to expect, upon the extremest 

 flood theories (such as are being more than revived at present 

 in the pages of the Geological Magazine), that they should 

 follow, upon a gigantic scale, the arrangements now presented 

 upon a smaller one, scooping vast scars at the larger turns of 

 valleys and accreting immense gravel banks opposite them, 

 based on blocks of gigantic size. The effects of unwonted 

 floods upon our modern valleys is no criterion of what would 

 happen if great floods were not the exception but the rule. 

 At the present day a large flood spreads pure devastation in 

 a valley unprepared for it. It is like a force of 1000 horse- 

 power brought to bear upon a machine calculated for but one 

 hundred. The machine is shaken to pieces. But let the 

 larger force be reserved for the greater machine, and all will 

 go according to the principles of construction, and the 

 ordinary laws of motion. 



The opinion seems to be gaining ground among the younger 

 oeologists of America, that in North America, as in Britain, 

 there was but one short torrential period which swept the 

 valley-banks and left bare slopes for subsequent river-action 

 to terrace. And the first question to be answered by the 

 student of river terraces is being recognised as this: "In 

 how far has the subsequent action been unaided river-action, 

 resulting in terraces not opposite ? " It is not allowable to 

 have recourse to coast elevation, or climatic changes, or 



* Professor Green remarks that the Yorkshire river-gravels are also coarser 

 in the lower part of each gravel-bed. This seems to apply to river-gravels of 

 all ages (" Geol. of the Yorkshire Coaltield," Mem. Geol. Survey, p. 784). See 

 also Figs. 9 and 10. 



