‘ ““~ 
ms . ~ ; “ are 
~ ~ 
418 - DYNAMICAL GEOLOGY. —_[Boox ITI. _ 
Hardly anything has yet been done in the way of actual ~ 
measurement of the rate of erosion by different glaciers. An 
approximation to the truth might be obtained from the abundant 
fine sediment which, giving the characteristic milky turbidity to all 
streams that escape from the melting ends of glaciers, is an index of 
the amount of this erosion. The average quantity of sediment — 
discharged from the melting end of a glacier during a year, having — 
been estimated, it would be easy to determine its equivalent in the 
precise fraction of a foot of rock annually removed from the area 
drained by the glacier. From the end of the Aar glacier (which 
with its affluents is computed to have an area of 60 square kilo- 
metres, and is therefore by no means one of the largest in Switzer- 
land) it has been estimated that there escape every day in the © 
month of August 2 million cubic metres (440 million gallons) of 
water, containing 284,374 kilogrammes (280 tons) of sand. Mr. 
A. Helland has computed that from the Justedal glacier, Norway, | 
one million kilogrammes of sediment are discharged in a July day, 
and that the total annual discharge from the ice-field, 830 square 
miles in area, amounts to 180 millions of kilogrammes, besides 
13 million kilogrammes of mineral matter in solution. Taking the — 
specific gravity of the suspended matter at 2°6, he finds that the 
basin of the glacier loses 69,000 cubic metres of solid rock every 
year, or a cubic mass measuring 41 metres on the side, There is 
some difficulty, however, in determining what proportion of the 
sediment may have been washed in below the ice by streams issuing 
from springs and melted snows. Estimates of the work done by 
glaciers, so far as based upon the amount of sediment discharged by 
them, may consequently be rather over the truth. 
§. 6. Oceanic Waters. 
The area, depth, temperature, density, and composition of the 
sea having been already treated of (Book II.), we have now to 
consider its place among the dynamical agents in geology. In this 
relation it may be studied under two aspects: Ist, its movements, 
and 2nd, its geological work. 
I. Movements.—(1.) Tides.—These oscillations of the mass of 
the oceanic waters caused by the attraction of the sun and moon 
require notice here only as regards their geological bearings. In a 
wide deep ocean the tidal elevation probably produces no perceptible 
geological change. It passes at a great speed; in the Atlantic its 
rate is 500 geographical miles an hour. But as this is merely the 
passing of an oscillation whereby the particles of water are gently 

basin-shaped receptacles in solid rock through the operations of superficial weathering 
—a process which may account for many rock-basins that have subsequently had their 
decomposed rock swept out of them by ice. 
» Aftryk ur Geol. Féren, Stockholm Férhandl. 1874, No. 21. Band ii. No. 7, . 


