the Distributiun of Life and of Rucks. 407 



river will be chiefly the mud [i. e. the felspatliic detritus) of the 

 watershed. 



If both forms of denudation go on together (as is often the 

 case), the visible result is that the clay-band, as it approaches 

 the river-mouth, extends out in a fan-shaped form, thinning off 

 the further it is removed from the land ; the great difference 

 between the two sets of clays being that, whereas the coast-day 

 has its greatest extent in the line of the coast, the river-clay has 

 its greatest extent in the line of the river, i. e. in a line at riglit 

 angles to the coast. 



Since all plutonic rocks are not granites, it may be that, in 

 place of mica, there will be a more easily decomposed horn- 

 blende. Many forms of amphibole, like some kinds of mica, 

 contain much iron. There can be no doubt that many fer- 

 ruginous sandstones owe their iron to the decomposition of 

 mica after deposition, just as it seems probable that many clay- 

 ironstones were formed partly from the decomposition of horn- 

 blende at the time of deposition. Two other important consti- 

 tuents are lime and magnesia; and I know of no origin for these 

 substances in nature except the plutonic rocks. Lime is to a 

 great extent soluble in water, under sea-shore conditions, and is 

 not precipitated by evaporation in an appreciable form where 

 there are other deposits foiming; hence it is that we usually 

 find limestones near to shores where there is no denudation, 

 and far out at sea beyond the limits of sedimentary deposits. 



6. If, therefore, we find a magnesiau limestone, it will be a 

 reasonable inference that, if it could be followed over the old 

 sea-bottom, it would merge into a clay on approaching land, 

 that the clay would probably pass into a clay-ironstone, and 

 this latter into a sandstone, beyond which must be an area 

 without any deposit synchronous with these, which, however, 

 would coexist in the same geological time, though of such dif- 

 ferent mineral characters, and these retained only under such 

 limiting circumstances. 



7. If beds have been already arranged in this order, which 

 may be called the necessary sequence of rocks, and a clitf" of them 

 comes to be denuded, they are once more resolved into their 

 elementary substances, and spread out as before. The recon- 

 struction, however, may be generally detected by the pebbles 

 and, it may be, extraneous fossils that it will contain. 



8. Under ordinary circumstances the river-clay is distinguished 

 from the shore-clay by its immense thickness; for, concentrating 

 to a point the clay of a large area, it accumulates more rapidly 

 than that resulting from tidal denudation ; moreover it is more 

 likely than any other kind of deposit to be continuous in the 

 same area through several geological periods. The river-sand 



28* 



