Correspondence — Colonel George Greenwood. 191 



the same is true of the Mastodon of the Norfolk stone-bed. The re- 

 mains of the Forest-bed are in the hands of Mr. Boyd Dawkins, who 

 doubtless will not allow them to be mixed up with Crag or Bone- 

 bed specimens. E. Eat Lankesteb. 



Hampstead. 



SUGGESTIONS ABOUT DENUDATION". 

 Sir, — Your number of this month (p. 109) contains a clever paper 

 by Mr. Kinahan. With one exception, I agree with everything that 

 he has said. The exception relates to what Mr. Mackintosh has 

 dubbed " My hard-gorge and soft- valley theory." I think that Dr. 

 Hooker's terraces are patches of alluvial plains (or river haughs) 

 sliced into terraces, and not fiUed-up lakes. Alluvial plains, pro- 

 perly so called, are deposited by the overflow of rivers upon flat dry 

 ground, and not in hollows like filled-up lakes. Take the engraving 

 of Dr. Hooker's terraces. On the left of the river, as you look at it, 



Diagram of the Glacial Terraces at the Fork of the Yangma Valley (copied, slightly reduced in 

 size, from Dr. Hooker's Himalayan Journals, vol. i. p. 219). 



are four terraces. Number them 1, 2, 3, 4 from the river. No. 1 is 

 now being formed in precisely the same way as all alluvial plains, 

 and as all the preceding terraces have been formed. That is, by 

 deposit from the overflow of the river on to the dry flat surface 

 of the terrace, which also receives the waste of the sides of 

 the valley and of the old terraces. No. 2 forms the banks of the 

 river when in flood, and is vanishing now in precisely the same way 

 as the preceding terraces have vanished. That is, the flooded river 

 pulls the loose banks down, till No. 2 is driven against the side of 

 the hill as No, 3 has been driven there. No. 1 then extends to the 

 hUl-side, and is added to by every flood till the bed of the gorge is 

 lowered. Then No. 1 shares the fate of No. 2, 3, 4, and a new 

 alluvium is formed at a lower level and at the expense of No. 1. 

 Mr. Kinahan asks " what causes the barrier ?" Any comparatively 

 hard strata which cross the stream below softer strata. Even the 



