Correspondence — Colonel George Greenwood. 191 



come wlien it could no longer overflow its banks. It would tlien in 

 floods tear its banks down instead of building them up, and the 

 banks would recede from the river in the form of two parallel 

 terraces. But when the river had cut its banks down to the level 

 at which it could again overflow them, it would again deposit on 

 them, and would form a new alluvium at the new level of the river. 

 If the rising of the land continued, this new alluvium, like the old 

 one, would recede as two parallel terraces ; and so, step by step, 

 would be formed, as long as the rising of the land continued, whether 

 the rising "was gradual or sudden. I have mentioned such terraces 

 at Loch Eanza in Arran, in the Atlienceum, 22nd July, 1865. 



So far in reference to marine alluvial plains. With regard to inland 

 patches of alluvial plains and their terraces, their formation has 

 nothing to do with the rising of the land, and nothing to do with 

 the comparative level of the sea and land. They result only from 

 the different hardness of the different strata of the same valley. 

 That is, directly as the strata are hard (owing to the retarding of 

 atmospheric disintegration and erosion), the valley is narrow, and 

 assumes the character of a gorge with falls or rapids in the river. 

 Directly as the strata are soft, the valley is worn back ■vi'ide and 

 flat.^ Eain floods checked at the gorge then overflow the flat, and 

 deposit an alluvium, as the falls or rapids of the hard gorge or 

 narroios sink from erosion, the bed of the river in the soft valley 

 above sinks also, till a time comes when the river can no longer 

 overflow the alluvium which it has formed. And the sinking of the 

 bed of the river in the inland alluviums produces the same effects as 

 the rising of the land does on the marine alluviums. That is, 

 alluvium after alluvium is formed one below the other, and in suc- 

 cession driven back as parallel terraces against the hill-side. 



If my theory is true, this must be going on now in Norway. 

 Thatis, there must be alluviums (not "basins" orpermanent "water- 

 surfaces ") forming now below the ancient terraces at the level at 

 which each river overflows above each gorge or rapid. These 

 actually progressing alluviums may be seen throughout nature. 

 They may be seen in Lyell's engi-aving of the Parallel Eoads of 

 Glen Koy, in Lord Milton's and Cheadle's terraces of the Eraser 

 river, Geological Magazine, 1867, Vol. IV., p. 206, and in Hooker's 

 terraces of the Yangma, ibid, p. 208. 



Doubtless your Magazine will be seen by Professor Kjerulf. Will 

 he do me the favour to state whether this is so or not? 



George Greenwood, Colonel. 

 Brookwood Park, Aleespoed, 

 10 th February, 1871. 



^ This is what Mr. Mackintosh has called Colonel Greenwood's " Hard gorge and 

 soft valley theory." 



