RAIN-WASH. 273 



RAIN-WASH. 



Not long ago I was walking over Middleton Moor in Derbyshire, an 

 elevated exposed tract of land lying a mUe or two north-east of Dovedale. 

 The most elevated points of the Moor — Arbor Low, Lean Low, &c. — rise 

 from 1,200 to 1,300 feet above the sea, and he near its margins, whilst 

 the centre is depressed. After walking over the limestone rock, barely 

 covered with short grass, which constitutes the gi'eater part of the 

 district, I found in a hollow near a farm house a considerable spread of a 

 light red loaraj^ deposit, which had evidently been largely dug, probably 

 to spread over and improve the neighbouring land. It appeared to be 

 fi'om six to twelve or more feet in thickness, and contained no stones or 

 fragments of foreign rocks. I had httle more than a passing glance at 

 this red clay, but it struck me that it was an interesting instance of the 

 chemical action of rain water on the typical rock of this district — the 

 Carboniferous or Mountain Limestone. 



The Carboniferous Limestone contains about ninety per cent, of 

 carbonate of lime, and, in addition, some silicate of alumina, silica, oxide 

 of iron, &c. Now, the rain water in falhng through the atmosphere 

 dissolves out of the latter a little carbonic acid gas, and this combines 

 with the carbonate of lime, forming a bi-cavbonate, which is soluble 

 in water, and which is consequently carried off in solution by the latter, 

 thus causing the " hardness" of the water of limestone regions. The other 

 ingredients of the hmestone, however, are left behind. Slowly and gi-adually 

 do they descend the hill sides. On the veiy hill-tops the bare rock 

 peeps forth, but the slopes are covered with a few inches of debris, 

 over and through which the rain water courses, gradually abstracting 

 the soluble part, the carbonate of lime, and mechanically " moving 

 on" the other ingredients, until they ai'rive at the lowest point. I 

 noted that all the slopes of a given inclination had a terraced or step- 

 hke appearance, the lines running regularly, as if ruled at intei-vals of 

 a few feet. If it be a valley thi-ough which a river runs, the insoluble sub- 

 stances on reaching the bottom are then carried off by the stream, whose 

 waters they make muddy and turbid ; but on the limestone moors most of 

 the water percolates through the rock, or disappears down swallow-holes, 

 leaving the residue in the surface hollows. Here it accumulates. Most of it 

 is clay, but there is some sand, and the whole is tinged red by the oxide 

 of iron, which, by exposure to the air and water has been raised, if it 

 were not previously in that condition, to the state of the hyd rated 

 peroxide. If we could measure the cubical contents of such a deposit, 

 and also by analysis determine accurately the composition of the rock 

 from which it was derived, we might obtain some interesting results as 

 to the amount of denudation of the district in recent times. We might, 

 perhaps, also obtain some idea of the rate of denudation, and the time 

 which it had taken to form such deposits. In connection with this, the 

 average rainfall might be made to jield results in aid. In the south of 

 England the clay-with-flints, which covers part of the chalk downs has 



