ALLUVIAL DEPOSITS. 513 



It must, however, be borne in mind that as the source of a river is so much 

 higher than its mouth, so are the deposits which it has formed at similar periods 

 much higher in elevation as we proceed upwards along its banks ; therefore these 

 divisions apply only to the relative heights of deposits at particular localities 

 in a valley. 



Great expanses of river-gravel are generally formed in districts where there is 

 much old marine or glacial gravel, from which it has been largely derived : the 

 stones therefore are interesting as relics of various epochs. In some instances, as 

 before mentioned. Glacial gravel may merge into River gravel in the lower 

 portions of a mountain valley ; but in many cases the wide-spread valley-deposits 

 appear too extensive to have been formed by the modern rivers. In Prof. 

 Prestwich's opinion the greater excavating powers of the rivers in the early part of 

 the Post-Pliocene period, as compared with those of their representatives at the 

 present day, were mainly due to a more rigorous winter climate, probably accom- 

 panied by a more abundant rain-fall and a greater tendency to floods. ' He thus 

 admits a kind of Diluvial action, as did also Mr. A. Tylor, who first suggested that 

 at that time there was a ' Pluvial Period.' ^ This condition may have arisen from 

 the elevation of the land. In the formation of valleys it must also be remembered 

 that rivers pursue a serpentine course, and erode now on one side and now on the 

 other, as evidenced by the alternate accumulations of gravel that bound their 

 courses. (See sequel. ) 



Contortions in gravel are attributed by Mr. Prestwich to river-ice grounding on 

 the soft deposits, while some of the larger blocks of stone in river-gravel may have 

 been transported by river-ice. 



Iron-pan. — Beds of gravel and sand of all ages contain much oxide of iron, and 

 this is largely taken up by waters that filter through them, and when arrested by 

 an impervious stratum the iron-oxide is deposited, and the gravel and sand become 

 cemented into a hard rock or conglomerate known as iron-pan. (See p. 463.) 



Bog Iron-ore. — In many instances, as remarked by Dana, the ferruginous matter 

 is washed out of the deposits into low places or marshes, where it forms beds of 

 cellular limonite, called "Bog Iron-ore." Such beds often contain nuts and 

 leaves, petrified by the oxide of iron. The iron, when carried by the waters, is in 

 solution as bicarbonate, or combined with organic acids derived from the soil. 

 The change to limonite takes place where the waters have a chance to stand and 

 evaporate. In this way vast beds of ore have been made.^ Bog Iron-ore is also 

 formed to some extent by Diatoms [GalUoncUa fcrnigiuea), which according to 

 Ehrenberg have the power of separating iron from water, and depositing it as 

 hydrous peroxide within their siliceous framework.* Sands and gravels are some- 

 times coloured black, the fragments being coated with oxide of manganese ; in 

 other cases artificial impurities have produced a similar discolouration, which is 

 generally due to sulphide of iron. ^ 



Gravel of different ages is largely dug for road-mending, etc. ; and some of the 

 more pebbly varieties are employed for making concrete. 



Pleistocene Alluvial Deposits. 



These deposits are not always to be readily distinguished from 

 those of Recent age, except by the organic and other remains 



1 Phil. Trans. 1864, pp. 251, 252 ; Rep. Brit. Assoc, for iSSo, Trans, of Sects. 

 p. 581 ; see also De la Beche, Geol. Manual, p. 30 ; Phil. Mag. 1829 (2), vi. 

 243 ; Conybeare, Proc. G. S. i. 148. On the subject of River Terracing, see 

 Hugh Miller, jun., Roy. Phys. Soc. Edin. 1883, p. 263. 



3 Phil. Mag. April, 1853 ; G. Mag. 18S1, p. 525 ; H. H. Howorth, G. Mag. 

 1883, pp. 356, 413; Godwin-Austen, Q.J. vii. 130. 



^ Manual of Geology, ed. 2, 1874. 



* A. Geikie, Text-Book of Geology, ed. 2, p. 175 ; Lyell, Principles of Geology, 

 ed. II, vol. ii. p. 50S. 



^ W. H, Hudleston and F. G. H. Price, P. Geol. Assoc, iii. 53. 



