the Distribution of Life and of Rocks. 407 
river will be chiefly the mud (7. e. the felspathic 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-clay 
has its greatest extent in the line of the coast, the river-clay has 
its greatest extent in the line of the river, 7. e. in a line at right 
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 forming; 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 magnesian 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 cliff 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 
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