16 LYELL'S ELEMENTS OF GEOLOGY. 



Aqueous Origin of certain Rocks. 



In the estuaries of large rivers, such as the Ganges 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 colour, and 

 are occasionally characterized by containing drift-wood or shells. 

 The shells may belong to species peculiar to the river, but are 

 sometimes those of marine testacea, washed into the mouth of 

 the estuary during storms. 



The annual floods of the Nile in Egypt are well known, and 

 the fertile deposit of mud which they leave on the plains. This 

 mud is stratified, the thin layer thrown down in one season 

 differing slightly in colour from that of a previous year, and 

 being separable from it, as has been observed in excavations at 

 Cairo, and other places.* 



When beds of sand, clay, and marl, containing shells and 

 vegetable matter, are found arranged in the same 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 example, at vari- 

 ous heights and depths in the earth, and often far from seas, 

 lakes, and rivers, we meet with layers of rounded pebbles com- 

 posed of different rocks mingled together. They are like the 

 pebbles formed in the beds of torrents and rivers, which are car- 

 ried down into the sea wherever these descend from high grounds 

 bordering a coast. There the gravel is spread out by the waves 

 and currents of the ocean over a considerable space ; but during 

 seasons of drought the torrents and rivers are nearly dry, and 

 have only power to convey fine sand or mud into the sea. Hence, 

 alternate layers of gravel and fine sediment accumulate under 

 water, and such alternations are found by geologists in the inte- 

 rior of every continent.f 



If a stratified arrangement, and the rounded forms of pebbles, 

 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 ^fossils, so abundantly 

 included in the earth's crust. By a fossil is meant any body, or 

 the traces of the existence of any body, whether animal or vege- 



* See Silliman's Amer. Journ. of Sci. vol. xxviii. 1835 ; also Principles of 

 Geology, Index, " Nile, " Rivers," &c. 



t See Principles of Geology, by the author ; refer to Magnan,' and ' Conglo- 

 merates, in the Index. 



