ALLUVIUM. 



77 



Steffens is of opinion that this mineral, and the selenite 

 with which it is accompanied, are formed by the 

 decomposition of iron pyrites; the sulphuric acid 

 thus formed is supposed to unite with the lime and 

 alumina; with the lime it forms sulphate of lime or 

 selenite, and with the alumina an alum, with a super- 

 abundance of alumina. 



ALLUVIUM. A term employed by geologists to 

 designate those accumulations of sand, earth, gravel, 

 and other materials brought down by the passage of 

 running streams, or deposited by the action of the 

 waves of the ocean. 



Water is one of the most active agents in the changes 

 which are continually taking place on the surface of our 

 globe. Its action in the form of rain, and even in the 

 almost invisible vesicles contained in the atmosphere, 

 (hough slow, is equally certain in its operation with 

 that of the rushing torrents which were let loose 

 when the foundations of the great deep were broken 

 n |). Hard rocks are thus gradually disintegrated, and 

 each change of temperature tends to separate a new 

 layer from many stony bodies which bid defiance to all 

 the ordinary processes employed by human ingenuity. 



Alluvial soils, generally speaking, occupy the lowest 

 and most level tracts of country; and when viewed in 

 connexion with rivers, point out in the most striking 

 manner the original level of their waters. Thus we find 

 that when the level of a lake, or river, has sunk in con- 

 sequence of spontaneous or artificial draining, the allu- 

 vial soil which has been deposited will mark a higher 

 line of level than its surface. 



The alluvial formations constitute the great mass 

 of the earth's surface. They may be divided into two 

 species ; namely, those deposited in the valleys of 

 mountainous districts, and those deposited upon flat 

 lands. 



The first species consists of sand, gravel, &c., which 

 constituted the more solid parts of the neighbouring 

 mountains, and which remained when the least solid 

 parts were washed away. They sometimes contain 

 ores (especially gold and tin) and the alluvial soil is 

 washed, in order to separate these ores. On moun- 

 tain plains there are beds of loam. The second species 

 of alluvial deposits, or that which occupies the flat 

 land, consists of loam, clay, sand, turf, and various 

 fossil remains. Bituminous wood and iron ore 

 also occur. The sand contains various species of 

 metals. 



To render our account of this subject more intelli- 

 gible, in a geological point of view, it may be advis- 

 able to notice, somewhat in detail, the Huttonian 

 opinion of the origin of alluvial deposits, in connexion 

 with the formation of our globe. 



Dr. Hutton does not go back to chaos to lay the 

 foundation of his habitable world ; nor does he bor- 

 row much assistance in constructing his fabric from 

 chemical attractions. He rests upon a pre-existing 

 continent, out of the ruins of which our present dry 

 land was formed and arranged principally by me- 

 chanical means. The portion of the globe which we 

 now possess, was, according to his hypothesis, the 

 bottonvof the sea, and the previous continents having 

 sunk into the depths of the ocean, new land rose to 

 supply their place, which thus became the present 

 terra firma ; and, k-st we should be alarmed at the 

 recurrence of a similar catastrophe, we are told that 

 it will be followed by a slow but similar renovation. 

 Thus, as one continent descends another rises, like 

 the opposite scales of a balance ; and, in the resources 



of the system, that order of organic nature is sup- 

 posed to be traced by which the continued existence 

 of the different races is secured, not by the perpetuity 

 of the individual, but by the successive reproduction 

 of the kind. Our present world is thus one in an in- 

 definite series of worlds which have existed in times 

 past, and which are destined in future to appear ; 

 and all the less obvious or more striking changes 

 which we witness are but steps in the progress 

 of mighty revolutions, to which the imagination 

 could set no limits, either with regard to duration or 

 magnitude. 



Dr. Hutton lays down as a certain position, that the 

 solid parts of our earth are suffering decay from the 

 action of the elements ; that the portions detached from 

 the more elevated ground are carried by the operation 

 of water to the lower levels. Thus forming the alluvial 

 masses of our globe. He conceives, that those por- 

 tions which reach the ocean are acted upon by tides 

 and currents, which arrange what is carried within 

 their influence in layers along the bottom of the sea. 

 Thus every river, every brook, every stream of water 

 that we see, descends towards the ocean, charged with 

 some portions of the surface over which it flows. All 

 the soil and softer parts on which our plants are pro- 

 duced, have been confessedly loosened by water, and 

 may be ultimately transported by it to the lowest 

 levels of the same element. The strata of our dry 

 land have all been thus carried from a pre-existing 

 one, and arranged by the ocean, which then covered 

 it. It may, however, be proper to remark, that effects 

 have been attributed to the action of running streams, 

 much beyond what their mechanical operation would 

 at all seem to warrant. We have seen that Dr. 

 Hutton supposes that the materials of all the strata 

 are the debris of a former world ; that they have been 

 detached from it by the operation of the elements ; 

 carried, by the agency of water, to the ocean ; and 

 there spread in regular order over its bottom by the 

 same power. It is true that most rivers flow towards 

 the ocean, charged with a part of the soil or softer 

 rock which border their channels ; but it is not so 

 true that they carry all this burden to their ultimate 

 destination. A great part of it is deposited on their 

 banks, or in the hollows of their courses ; and much 

 of what reaches the sea goes to form bars, or, being 

 driven back to the shore, makes an addition to the sea- 

 coast. It is evident that a small portion only can reach 

 the ocean ; and if, as Dr. Hutton himself observes, the 

 description which Polybius has given of the Pontus 

 Euxinus, with the two opposite shores of the Bosphorus, 

 the Mseotis, the Propontis, and the port of Byzantium, 

 is as applicable to the present state of things as it 

 was at the writing of that history ; if the isthmus of 

 Corinth is apparently the same at present as it was 

 2,000 or 3,000 years ago ; if Scylla and Charybdis 

 remain now, as they were in ancient times, rocks 

 hazardous for coasting vessels ; if the port of Syra- 

 cuse, and the fountain of Arethusa, the water of which 

 the ancients divided from the sea with a wall, do not 

 seem to be altered ; and if, on the coast of Egypt, 

 we find the rock on which was formerly built the 

 famous tower of Pharos ; and, at the eastern extre- 

 mity of the port Eunoste, the sea-bath cut in the 

 solid rock on the shore, to all appearance the same 

 at this day as they were in ancient times ; if such be 

 the extreme slowness of the disintegration, the re- 

 flection is obvious, that, admitting it, a duration will 

 be allowed to the world infinitely beyond our con- 



