Ch. I] AQUEOUS ROCKS. 3 



For, whenever a running stream charged with mud or sand, has its ve- 

 locity checked, as when it enters a lake or sea, or overflows a plain, the 

 sediment, previously held in suspension by the motion of the water 

 sinks, by its own gravity, to the bottom. In this manner layers of mud 

 and sand are thrown down one upon another. 



If we drain a lake Avhich has been fed by a small stream, we frequently 

 find at the bottom a series of deposits, disposed with considerable regu- 

 larity, one above the other ; the uppermost, perhaps, may be a stratum 

 of peat, next below a more dense and solid variety of the same material ; 

 still lower a bed of shell-marl, alternating with peat or sand, and then 

 other be,ds of marl, divided by layers of clay. Now, if a second pit be 

 sunk through the same continuous lacustrine formation, at some distance 

 from the first, nearly the same series of beds is commonly met with, yet 

 with slight variations ; some, for example, of the layers of sand, clay, or 

 marl, may be wanting, one or more of them having thinned out and 

 given place to others, or sometimes one of the masses first examined is 

 observed to increase in thickness to the exclusion of other beds. 



The term "formation" which I have used in the above explanation, 

 expresses in geology any assemblage of rocks which have some character 

 in common, whether of origin, age, or composition. Thus we speak of 

 stratified and unstratified, freshwater and marine, aqueous and volcanic, 

 ancient and modern, metalliferous and non-metalliferous formations. 



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 pe- 

 riodical 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 color, 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, especially in la- 

 goons nearly separated by sand-bars from the ocean, deposits are often 

 formed in which brackish-water 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-deposited sediment is 

 stratified, the thin layer thrown down in one season differing slightly 

 in color 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 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 resem- 

 blance. Thus, for example, at various heights and depths in the earth, 

 and often for from seas, lakes, and rivers, we meet with layers of rounded 



* See Principles of Geology, by the Author, Index, "Nile," <v Rivers," <i"C. 



