28 
ELEMENTS OF GEOLOGY. 
age, or composition. Thus we speak of stratified and unstrat* 
ified, fresh-water and marine, aqueous and volcanic, ancient 
and modern, metalliferous and non-metalliferous formations. 
In the estuaries of large rivers, such as the Gauges and 
the Mississippi, we may observe, at low water, phenomena 
analogous to those of the drained lakes above mentioned, 
but on a grander scale, and extending over areas several 
hundred miles in length and breadth. When the periodical 
inundations subside, the river hollows out a channel to the 
depth of many yards through horizontal beds of clay and 
sand, the ends of which are seen exposed in perpendicular 
cliffs. These beds vary in their mineral composition, or col¬ 
or, or in the fineness or coarseness of their particles, and 
some of them are occasionally characterized by containing 
drift-wood. At the junction of the river and the sea, espe¬ 
cially in lagoons nearly separated by sand-bars from the 
ocean, deposits are often formed in which brackish and salt¬ 
water shells are included. 
In Egypt, where the Nile is always adding to its delta by 
filling up part of the Mediterranean with mud, the newly de¬ 
posited sediment is' stratified^ the thin layer thrown down in 
one season differing slightly in color from thiat of a previous 
year, and being separable from it, as has been observed in 
excavations at Cairo and other places.^ 
W^hen beds of sand, clay, and marl, containing shells and 
vegetable matter, are found arranged in a similar manner in 
the interior of the earth, we ascribe to them a similar origin; 
and the more we examine their characters in minute detail, 
the more exact do we find the resemblance. Thus, for exam¬ 
ple, at various heights and depths in the earth, and often far 
from seas, lakes, and rivers, we meet with layers of rounded 
pebbles composed of flint, limestone, granite, or other rocks, 
resembling the shingles of a sea-beach or the gravel in a tor¬ 
rent’s bed. Such layers of pebbles frequently alternate with 
others formed of sand or fine sediment, just as we may see 
in the channel of a river descending from hills bordering a 
coast, where the current sweeps down at one season coarse 
sand and gravel, while at another, when the waters are low 
and less rapid, fine mud and sand alone are carried sea¬ 
ward, f 
If a stratified arrangement, and the rounded form of peb¬ 
bles, are alone sufficient to lead us to the conclusion that 
certain rocks originated under water, this opinion is farther 
confirmed by the distinct and independent evidence of fos- 
* See Principles of Geology, by the Author, Index, “Nile,” “Rivers,”etc. 
t See p. 44, Fig. 7. 
