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Show 161) ISLAND LIFE. (PART I. h Let us t en m. qm.r e fi rs t I'nto the character of the evidence c. we should expect to find of such changes of .climate, if. they d h ll then be in a better positiOn to est1mai.e have occurre ; we s a · . at its proper value the evidence that actually ex1~ts, and, after g1· v·m g 't d e weiaht to arrive at some conclusiOn as to the 1 u o ' . . theory that best explains and harmomses It. Effects of Demtdation in dest?·oying the evidence of remote Glacial Epochs.-It may be supposed, that if earlier glac.ial epochs than the last did really occur, .we ought to m~et w1th some evidence of the fact correspondmg to , t.hat wh1ch has satisfied us of the extensive recent glaciation of the northern hemisphere ; but Dr. Croll and other writeFs h~ve ably argued that no such evidence is likely to be found. It 1s now generally admitted that sub-aerial denudation is a much more powerful aaent in lowerina and modifving the surface of a country than n o J was formerly supposed. It ~as in fact been proved to be so powerful that the difficulty now felt is, not to account for the denudation which can be proved to have occurred, but to explain the apparent persistence of superficial features which ought lung ago to have been destroyed. A proof of the lowering and eating away of the land-surface which every one can understand, is to be found in the quantity of solid matter carried down to the sea and to low grounds by rivers. This is capable of pretty ~ccurate measurement, and has been so measured for several rivers, large and small, in different parts of the world. The details of these measurements will be given in a future chapter, and it is only necessary here to state that the average of them all gives us this resultthat one foot must be taken off the entire surface of the land each 3,000 years in order to produce the amount of sediment and matter in solution which is actually carried into the sea. To give an idea of the limits of variation in different rivers it may be mentioned that the Mississippi is one which denudes its valley at a slow rate, taking 6,000 years to remove one foot; while the Po is the most rapid, taking only 729 years to do the same work in its valley. The cause of this difference is very easy to understand. A large part of the area of the Mississippi CHAP. IX.] ANCIENT GLACIAL EPOCIIS. 1G7 basin consists of th e a l mos t ra.m 1 e ss prau. 1. e and desert reaw. ns of th.e west ' whil e 1·t s sour.c es are 1· n comparati.v ely an.d moou ntam. s With scanty snow-fields, or in a low forest-clad plateau. The Po, on the other hand, is wholly in a district of abundant rainfall while its sources are spread over a great amphitheatre of snow; Alps nearly .400 miles in extent, where the denuding forces are at a maximum. As Scotland is a mountain reaion of rather abundant rainfall, the denuding power of its rain: and rivers is probably rather above than under the average, but to avoid any possible exaggeration we will take it at a foot in 4,000 years. Now if the end of the glacial epoch be taken to coincide with the termination of the last period of high excentricity, which occurred about 80,000 years ago (and no geologist will consider this too 1ong for the changes which have since taken place), it follows that the entire surface of Scotland must have been since lowered an average amount of twenty feet. But over large areas of alluvial plains, and wherever the rivers have spread during floods, tbe ground will have been raised instead of lowered; and on all nearly level ground and gentle slopes there will have been comparatively little denudation; so that proportionally much more must have been taken away from mountain sides and from the bottoms of valleys having a considerable downward slope. One of the very highest authorities on the subject of denudation, Mr. Archibald Geikie, estimates the area of these more rapidly denuded portions as only one-tenth of the comparatively level grounds, and he further estimates that the former will be denuded about ten times as fast as the latter. It follows that the valleys will be deepened and widened on the average about five feet in the 4,000 years instead of one foot; and thus many valleys must ha.ve been deepened and widened 100 feet, and some even more, tiince the glacial epoch, while the more level portions of the country will have been lowered on the average only about two feet. Now Dr. Croll gives us the following account of the present aspect of the surface of a large part of the country:- "Go where one will in the lowlands of Scotland and he shall hardly find a single acre whose upper surface bears the marks of being formed by the denuding ageuts now in operation. He |