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Show 206 ISLAND LIFE. (PAR'!' f. contemplated can be granted. From a consideration of the possible sources of the beat of the sun, as well as from calculations of the period during which the earth can have been cooling to bring about the present rate of increase of temperature as we descend beneath the surface, Sir William Thomson concludes that the crust of the earth cannot have been solidified much longer than 100 million years (the maximum possible being 400 millions), and this conclusion is held by Dr. Croll and other men of eminence to be almost indisputable.1 It will therefore be well to consider on what data the calculations of geologists have been founded, and how far the vie·ws here set forth, as to frequent changes of climate throughout all geological time, may affect the rate of biological change. Denudation and Deposition of Strata as a measu1·e of Time.-The materials of all the stratified rocks of the globe have been obtained from the dry land. Every point of the surface is exposed to the destructive influences of sun and wind, frost, snow, and rain, which break up and '~ar away the hardest rocks as well as the softer deposits, and by means of rivers convey the worn material to the sea. The existence of a considerable depth of soil over the greater part of the earth's surface; of vast heaps of rocky debris at the foot of every inland cliff; of enormous deposits of gravel, sand, and loam; as well as the shingle, pebbles, sand, or mud, of every sea-shore, alike attest the universality of this destructive agency. It is no less clearly shown by the way in which almost every drop of running waterwhether in gutter, brooklet, stream, or large river-becomes discoloured after each heavy rainfall, since the matter which causes this discolouration must be derived from the surface of the country, must always pass from a higher to a lower level, and must ultimately reach the sea, unless it is first deposited in some lake, or by the overflowing of a river goes to form an alluvial plain. The universality of this subaerial denudation, both as regards space and time, renders it certain that its cumulative effects must be very great; but no attempt seems to have 1 Trans .. Royal Society of Edinburgh, Vol. XXIII. p.161. Quarterly JoU1'nal of Sctence, 1877. (Croll on the " Probable Origin and Age of the Sun.") CilAl'. x.] 'l'llE EA lt'l'Il'S AGK 207 been made to determine the magnitude of those effects till Mr. Alfred Tylor, in 1853,1 pointed out that by measuring the quantity of solid matter brought down by rivers (which can be done with considerable accuracy), we may obtain the amount of lowering of the land -area, and also the rise of the ocean level, owing to the quantity of matter deposited on its floor. A few years later Dr. Croll applied the same method in more detail to an estimate of the amount by which the land is lowered in a given period; and the validity of this method has been upheld by Prof. Geikie, Sir Charles Lyell, and all otir best geologists, as affording a means of actually determining with some approach to accuracy, the time occupied by one important phase of geological change. The quantity of matter carried away from the land by a river is greater than at first sight appears, and is more likely to be under- than over-estimated. By taking samples of water near the mouth of a river (but above the influence of the tide) at a sufficient number of points in its channel and at different depths, and repeating this daily or at other short intervals throughout the year, it is easy to determine the quantity of solid matter held in suspension and solution; and if corresponding observations determine the quantity of water that is discharged. the total amount of solid matter brought down annually may be calculated. But besides this, a considerable quantity of sand or even gravel is carried along the bottom or bed of the river, and this has rarely been estimated, so that the figures hitherto obtained are usually under the real quantities. There is also another source of error caused by the quantity of matter the river may deposit in lakes or in flooded lands during its course, for this adds to the amount of denudation performed by the river, although the matter so deposited does not come down to the sea. After a careful examination of all the best records, Professor A. Geikie arrives at the following results, as to the quantity of matter removed by seven rivers from their basins, estimated by the number of years required to lower the whole surface an average of one foot: I Philo.90])hicnl Magqrdne, April 1853. |