T. M. READE ON A DELTA IN MINIATURE, 265 
opposite the culvert. ‘The area of this delta I estimated at 430 
yards and the average thickness at 3 yards. 
Rake Brook.—On walking up one fork of the Rake Brook and 
coming down the other I found the brook-beds full of large Mill- 
stone-grit boulders; in places the rock in setw formed the bed of 
the brook. * 
The banks, largely cut down vertically, showed a drift composed 
entirely of the débris of the underlying rocks from large stones 
through boulders to gravel and sand; much of it seemed like sub- 
aerial detritus. On the top of this was a peaty soil of considerable 
thickness. in fact all the materials of the delta were there and 
nothing more. I observed no erratic stones. 
The Drainage-area.—On walking from one fork to the other I 
passed over the heath, which was extensively covered with the 
cotton grass, the beautiful tufts of downy cotton looking at a distance 
like white flowers. This heath, according to the 6-inch ordnance 
map, kindly furnished by Mr. Parry, constitutes about $ of the drain- 
age area; it can hardly be called a “ basin” as it occupies the side of 
the hill. The reservoir is about 550 feet above O.D., and the highest 
point of the watershed about 1200 feet above O.D. I estimated the 
drainage-area or “ gathering-ground” at 3,643,000 square yards 
=1-176 square mile. The delta I estimated at 6,306 cubic yards ; 
and as the time of accumulation was 27 years, the rate of removal 
or denudation over the whole surface was ,}, of an inch per annum 
or 1 foot in 5184 years. The mean rainfall of the Rake-Brook 
watershed was, during the last 10 years, according to information 
supplied me by Mr. Parry, 49°57 inches per annum. 
Observations.—No doubt some of the finer mud has been dis- 
tributed over the bottom of the reservoir and is not taken into 
account in my estimate. It is interesting to note that the rate is 
nearly the same as that of the Mississippi*. It is also perhaps more 
instructive to note what a close agreement there exists between the 
relative amounts of peaty matter and sand in the deposit and on 
the surface of the gathering-ground. The example also forcibly 
impresses on the mind the importance of causes at work, which, 
but for exceptional circumstances which direct our attention to 
their accumulated results, would not be noticed. Ii the reservoir 
had not been made, the matter would have passed out to sea through 
the river Douglas without notice; as it was, the matter intercepted 
seemed so exceptionally great for the time as to attract the attention 
of the engineer ; but, on measurement, we find that the rate of denu- 
dation accords well with what has been calculated in other ways. 
Being in miniature, we are enabled to directly inspect and explore 
all the processes of nature leading to the result, both on the gather- 
ing-ground and in the deposit itself. We also see very beautifully 
displayed those puzzling alternations of fine laminated beds and 
coarse sand with boulders resulting from the varying conditions in 
* See ‘Text book of Geology’ by Dr. Archibald Geikie: given at 1 foot in 
6,000 years. ‘The mean rainfall of the whole Mississippi valley is, according to 
data given by Humphreys and Abbott, about 30 inches. 
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