Correspondence — Col. George Greenwood. * 45 



THE UPPER INDUS BASIN. 



Sir, — Under this heading in the August Journal of the Geological 

 Society, Mr. Drew gives us an excellent collection of facts admirably 

 illustrated with drawings. In theorizing on these facts Mr. Drew 

 supports throughout the view which I had the honour to advance 

 in the Geological Magazine (Vol. IV. 1867, p. 205), under the 

 head of " Valley Terraces," namely, that the parallel terraces are the 

 remains of river alluviums, and that they are not, as is generally 

 held, shores of ancient lakes, or due to marine action, as is also 

 argued. But beyond this, Mr. Drew's facts cut entirely against his 

 own theories. That is, his facts prove that rain and rivers have 

 made, and aire still making, his side " fans," main river alluviums 

 and terraces, and that these are not (according to his theory) due to 

 the glacial epoch (page 470), or to any former " different state of 

 climate " (page 457). His facts also demonstrate that these inland 

 patches of river alluviums (which are not to be confused with those 

 open to the sea) are due to what Mr. Mackintosh has called "Colonel 

 Greenwood's hard gorge and soft valley theory." Every one of 

 Mr. Drew's flat alluviums is immediately above a hard gorge. And 

 this immediate alternation of gorge and flat alluvium is found not 

 only in the Indus valley, but throughout the wide wide world, 

 wherever a river crosses strata of different hardness. Directly as 

 the strata are hard is the narrowness of the gorge which the river 

 cuts. This gorge the river has difficulty to deepen, and atmospheric 

 disintegration and the erosion of rain have difficulty to widen. But 

 throughout the soft strata above the hard gorge the river cuts its 

 bed flat at the level of the gorge, and the atmosphere and the rain 

 easily make and widen a flat valley. The water of every rain flood 

 is then checked at the narrow gorge, overflows the flat valley, and 

 deposits an annually increasing alluvium on it. But as the allu- 

 vium rises by deposit, the bed of the gorge (and consequently of the 

 river above) sinks from erosion, and the time comes when the river 

 can no longer overflow. The confined flooded river then tears down 

 the soft alluvial banks of its own building, and eats them away to 

 the hill-sides, where they remain as parallel terraces. A new flat 

 and a new alluvium is then begun at the new level of the gorge. 

 This process has been repeated for what man may call an eternity 

 of time. It is still going on with unabated vigour, and will continue 

 to go on as long (as Mr. Drew says) " as there are mountains behind 

 to waste." 



This is the simple " open Sesame" of Mr. Drew's puzzle (p. 469), 

 " to account for rivers and streams being at one time denuders, at 

 another accumulators, and at a third time denuders again — in other 

 words, to discover why these streams shall at one time lower their 

 beds, cutting down through the rock, at another raise them by 

 depositing alluvium and rising upon it, and at last again lower their 

 beds by cutting down through that alluvium." 

 Brookwood Park, Alrbsford, Georgb Gkeenwood, Colonel. 



