ALLUVIUM. 
99 
observations in the same “ Survey,” that the Palaeozoic strata 
are from 20,000 to 30,000 feet thick. It is clear that such 
rocks, formed of mud and sand, now for the most part con¬ 
solidated, are the monuments of denuding operations, which 
took place on a grand scale at a very remote period in the 
earth’s history. For, whatever has been given to one area 
must always have been borrowed from another; a truth 
which, obvious as it may seem when thus stated, must be re¬ 
peatedly impressed on the student’s mind, because in many 
geological speculations it is taken for granted that the ex¬ 
ternal crust of the earth has been always growing thicker 
in consequence of the accumulation, period after period, of 
sedimentary matter, as if the new strata were not always 
produced at the expense of pre-existing rocks, stratified or 
unstratified. By duly reflecting on the fact that all de¬ 
posits of mechanical origin imply the transportation from 
some other region, whether contiguous or remote, of an 
equal amount of solid matter, we perceive that the stony ex¬ 
terior of the planet must always have grown thinner in one 
place, whenever, by accessions of new strata, it was acquir¬ 
ing thickness in another. 
It is well known that generally at the mouths of large riv¬ 
ers, deltas are forming and the land is encroaching upon the 
sea; these deltas are monuments of recent denudation and 
deposition; and it is obvious that if the mud, sand, and 
gravel were taken from them and restored to the continents 
they would fill up a large part of the gullies and valleys 
which are due to the excavating and transporting power of 
torrents and rivers. 
Alluvium.—Between the superficial covering of vegetable 
mould and the subjacent rock there usually intervenes in 
every district a deposit of loose gravel, sand, and mud, to 
which when it occurs in valleys the name of alluvium has 
been popularly applied. The term is derived from alluvia^ 
an inundation, or alluo^ to wash, because the pebbles and 
sand commonly resemble those of a river’s bed or the mud 
and gravel washed over low lands by a flood. 
In the course of those changes in physical geography 
which may take place during the gradual emergence of the 
bottom of the sea and its conversion into dry land, any spot 
may either have been a sunken reef, or a bay, or estuary, or 
sea-shore, or the bed of a river. The drainage, moreover, 
may have been deranged again and again by earthquakes, 
during which temporary lakes are caused by landslips, and 
partial deluges occasioned by the bursting of the barriers of 
such lakes. For this reason it would be unreasonable to 
