Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/305

Rh INFLUENCE or M.i.'.] preserved in a rainless or almost raiiiless climate. GEOLOGY 291 On the the aspect of a country in many ways :—(1) by chang- west of Europe isolated stacks and rocky islands in the sea ing forest into bare mountain, or clothing bare mountains are often seen to be white from the droppings of clouds of ; with forest ; (2) by promoting the growth or causing the sea birds ; but it is merely a thin crust, which is not allowed removal of peat-inosses ; (3) by heedlessly uncovering sand- to grow thicker in a climate where rains are frequent and dunes, and thereby setting in motion a process of destruc- heavy. IV. l[AN AS A GEOLOGICAL AGENT. N 0 survey of the geological workings of plant and animal life upon the surface of the globe can be complete which does not take account of the inﬂuence of nian—an inﬂuence of enormous and increasing consequence in physical geo- graphy, for man has introduced, as it were, an element of antagonism to nature. Not content with gathering the fruits and capturing the animals which she has offered for liis sustenance, he has, with advancing civilization, engaged in a contest to subdue the earth and possess it. His war- fare indeed has often been a blind one, successful for the moment, but leading to sure and sad disaster. He has, for instance, stripped off the woodland from many a region of hill and mountain, gaining his immediate object in the possession of their stores of timber, but thereby laying bare the slopes to parching droughts or ﬁerce rains. Countries once rich in beauty, and plenteous in all that was needful for his support, are now burnt and barren, or washed bare of their soil. It is only in comparatively recent years that he has learnt the truth of the aphoi'ism—“IIomo £'atm'(e minister et interpres.” But now, when that truth is coming more and more to be recognized and acted on, man’s influence is none the less marked. His object still is to subdue the earth, and he attains it, not by setting nature and her laws at deﬁance, but by enlisting her in his service. Within the compass of this article it is impossible to give more than merely a reference to this vast subject. The action of man may be witnessed on climate, on the ﬂow of water, on the charac- ter of the terrestrial surface, and on the distribution of life.1 1. On Oli'mate.—Human interference affects meteoro- logical conditioiis—(1) by removing forests and_ laying bare to the sun and winds areas which were previously kept cool and damp under trees, or which, lying on the lee side, were protected from tempests ; as already stated, it is supposed that the wholesale destruction of the woodlands formerly existing in countries bordering the Mediterranean has been in part the cause of the present desiccation of these districts; by drainage, the effect of this opera- tion being to remove rapidly the discharged rainfall, to lessen the evaporation, and thereby to diminish the rainfall and somewhat increase the general temperature of a country; by the other processes of agriculture, such as the transformation of moor and beg into cultivated land, and the clothing of bare hillsides with green crops or plantations of coniferous and hardwood trees. 2. On the Flow of ll'aler.—l. By increasing or diminish- ing the rainfall man directly affects the course of the waters over the land. 2. By his drainage operations he makes the rain to run off more rapidly than before, and thereby increases the ﬂoods in the rivers. 3. By wells, bores, mines, or other siibterranean works he interferes with the underground waters and consequently with the dis- charge of springs. 4. By eiiibaiiking rivers he conﬁnes them to narrow channels, sometimes increasing their scour, and enabling them to carry their sediment further seaward, sometimes causing them to deposit it over the plains and raise their level. 3- 0/l "16 »5'?U;/'ace of (lie L(mcl.———Man’s operations alter _1 The reader will find much suggestive matter in _Iarsh‘s illan and -Zl“5_1’7"’, 11 Work which, as its title denotes, specially treats of this suli_]c-St. A new and enlarged edition of this volume was published in 18; 4 under the title of The Earth as Jtlodiﬁcd by Human Action. tion which may convert hundreds of acres of fertile land into waste sand, or by prudently planting the dimes with sand- loving vegetation or pines, and thus arresting their landward progress ; (4) by so guiding the course of rivers as to make them aid him in reclaiming waste land, and bringing it under cultivation; (5) by piers and bulwarks, whereby the ravages of the sea are stayed, or by the thoughtless re- moval from the beach of stones which the waves had themselves thrown up, and which would have served for a time to protect the land; (6) by forming new deposits either designedly or incidentally. The roads, bridges, canals, railways, tunnels, villages, and towns with which man has covered the surface of the land will in many cases form a permanent record of his presence. Under his hand the whole surface of civilized countries is very slowly covered by a stratum, either formed wholly by him, or due in great measure to his operations, and containing many relics of his presence. The soil of old cities has been increased to a depth of many feet by the rubbish of his buildings; the level of the streets of modern Rome stands high above that of the pavements of the Caesars, and that again above the roadways of the early republic. Over cultivated ﬁelds his potsherds are turned up in abundance by t-he plough. The loam has risen within the walls of his graveyards, as gene- ration after generation has mouldercd there into dust. 4. On the Disla-2'butz'on o_f Lg'fc.——It is under this head, perhaps, that the most subtle of human inﬂuences come. Some of man’s doings in this domain are indeed plain enough, such as the extirpation of wild animals, the dimimition or destruction of some forms of vegetation, the introduction of plants and animals useful to himself, and especially the enormous predominance given by him to the cereals and to the spread of sheep and cattle. But no such extensive disturbance of the normal conditions of the distri- bution of life can take place without carrying with it many secondary effects, and setting in motion a wide cycle of changeand of reactionin the animal and vegetable kingdoms. For example, the incessant warfare waged by man against. birds and beasts of prey in districts given up to the chase leads sometimes to unforseeii results. The weak game is allowed to live, which would otherwise be killed off and give more room for the healthy remainder. Other animals which feed perhaps on the same materials as the game are by the same cause permitted to live unchecked, and thereby to act as a further hindrance to the spread of the protected species. But the indirect results of man’s interference with the 7-égime of plants and animals still require much pro- longed observation. From this brief and imperfect outline the reader may perceive that man takes an important place as a geological agent, and that in future ages the traces of his interference will introduce a new element of diﬁiculty into the study of geological phenomena. PART IV.——STRUCTL'PAL GEOLOGY, on THE .-mcriirizcrtnr. or THE EAr.Tii’s cr.L's'r. Having considered the nature of the materials constitute ing the crust of the earth, and the operation of the different agencies by which these materials are produced, arranged, and modiﬁed, we may now proceed to examine the st_ruct-lire of the crust itself with the view of marking how its com- ponent parts have been put together. Since by far the largest portion of the crust consists of sedimentary oi