ALLUVIUM. 



ALLUVIUM. 



ocean itself, hold a large quantity of earthy 

 matter in mechanical solution, which they de- 

 posit in their beds. The character of tin 1 MM!I- 

 ment is governed by the nature of the rocks 

 over which the waters flow; and the qu an tin- 

 depends partly upon the constitution of the 

 rocks, and partly upon the power of the water. 

 If the rock be easily destroyed, and a large 

 body of water flow over it with a considerable 

 velocity, the destructive effect will be great, 

 and much worn materials (detrf.tux) being 

 formed, the stream will have a thick and tur- 

 bid appearance. The same effect is frequently 

 produced by the discharge of a number of tri- 

 butary streams into a river, all of which accu- 

 mulate a greater or less quantity of the earths 

 over which they i!<>\v. 



The distribution of water at the present 

 time, and I more particularly refer to rivers, 

 is very different from that of former periods. 

 The majority of the valleys through which 

 rivers are now flowing, have been produced 

 by the action of water, which, running from 

 higher lands, has not only scooped them out, 

 but has spread over them the worn material 

 which it accumulates in its passage. By the 

 operations which have since been going on, 

 the waters have been collected together in 

 comparatively narrow channels of consider- 

 able permanency. On this account, the influ- 

 ence of water that flows over the portions of the 

 earth inhabited by terrestrial animals is great- 

 ly restricted; and the production of new beds 

 of rock or soil is rather an accidental than a 

 necessary consequence. 



But, although the influence of water has 

 been thus confined, all lands, and especially 

 the surfaces of mountainous districts, are un- 

 dergoing change, and the superficial covering 

 of one district is conveyed to another. The 

 showers of heaven are constantly sweeping 

 away the soil and decomposed rocks of the 

 uplands into the valleys, over which they are 

 transported by streams and rivers, the larger 

 and heavier particles falling to the bottom, the 

 smaller being united with the water in mechani- 

 cal mixture. That portion of earthy matter 

 which is carried away from a district by the 

 running water, is, as far as the district itself 

 is concerned, the most valuable, being the 

 superficial covering or soil, and would be for 

 ever lost to that portion of the earth inhabited 

 by man, were it not arrested in its passage to 

 the ocean, by deposition in the bed of the 

 river, or on those lands which the waters may 

 happen to overflow. 



It is well known to those who have visited 

 elevated districts, that many mountains are 

 already deprived of their soils, and are but the 

 skeletons of the earth, without covering or life. 

 By this action the valleys are in the process of 

 elevation, and the mountains of depression ; 

 and if we could conceive it to proceed without 

 limitation, we may imagine a time when all 

 the varieties of elevation and depression, which 

 HOW give beauty to the surface, will be de- 

 str<;'yed, and an entirely different condition of 

 the distribution of land and water will be 

 established. But, at the same time, it cannot 

 be denied that these changes, as far as they 

 have hitherto proceeded, have been advanta- 



geous to man, whatever might be their result 

 under the conditions to which we have alluded. 

 The mountainous regions are, from their ele- 

 vation, less suited to the progression of so- 

 ciety, so intimately connected with agricultu- 

 ral prosperity, than the plains. As we rise 

 above the level of the sea, the atmosphere be- 

 comes more rarified, and the cold more in- 

 tense, both of which are injurious to vegeta- 

 tion in general, and unsuited to promote the 

 comfort of animal life. The plains are, there- 

 fore, preferred by men when they congregate 

 together, and form societies. It cannot be 

 considered an unwise or unfit result, that the 

 lowlands should be enriched with alluvial 

 soils, produced by the destruction of the rocks 

 and natural soils of mountainous regions. It 

 is reported of Dioclesian, that he told his col- 

 league, Maximilian, he had more pleasure in 

 the cultivation of a few potherbs which, in the 

 gardens of Spalatro, grew in the soil that on 

 the top of Mount Hsemus had only produced 

 moss and dittany, than in all the honours the 

 Roman empire could confer. From the defini- 

 tion I have given of the word "alluvium," I 

 must include the gravels and sands that are of 

 recent formation among the alluvial deposits ; 

 but our attention is chiefly directed to the soils, 

 or those beds which are suited to sustain vege- 

 table life. It is true that the gravels may be 

 made available for the cultivation of some 

 plants, but the beds which are so used belong 

 rather to that class of rocks denominated dilu- 

 vinl by geologists, than to the deposits of which 

 we are speaking. 



If we trace the circumstances under which 

 alluvial soils are formed to their cause, we 

 shall find that they have their origin in the fall 

 of heavy rains, and the melting of snows, in 

 mountainous regions. The water, in its pas- 

 sage to the valleys, collects the superficial soil 

 and decomposed earthy material that lies in its 

 path, and transports them into the channels to- 

 wards which it flows. The streams that are 

 formed on the mountain slopes are generally 

 united together before they reach the plains, 

 and form impetuous torrents, overcoming all 

 obstacles, until their velocity is lost, when, in 

 their winding courses, they meet each other, 

 and form rivers. 



Rivers, in every part of their course, are 

 subject to inundation; when, throwing their 

 waters over a considerable space, they deposit 

 the earthy materials they have accumulated. 

 If such inundations had not occurred, the ac- 

 cumulated worn materials (debris) would have 

 been deposited in the bed of the river, or car- 

 ried into the lake or sea where the waters 

 themselves are discharged. There are abun- 

 dant instances on record of the filling up of 

 rivers by the worn materials (detritus), which 

 have been carried into their courses; and any 

 river of our own country will afford a limited 

 example of this result. Many rivers and es- 

 tuaries, which a few years since were navi- 

 gable, have ceased to be so on account of the 

 large amount of alluvial matter deposited in 

 their beds ; and many of our towns, which 

 were once populous and wealthy, have on this 

 account become poor and almost deserted. If 

 we would see the effect of the transport of worn 



