OIL. 201 



causes which segregated the rocks of igneous origin, appear to have 

 destroyed or removed one or several of their elements before their 

 new consolidation ; one of the most common deposites, sandstone or 

 grit, is almost wholly composed of grains of quartz, amidst which 

 particles of mica are frequently encountered ; but felspar is ex- 

 tremely rare. In the oldest sedimentary strata of the series, as in 

 the greywackes, the igneous elements are met with more complete, 

 and less altered. The structure of the calcareous rocks of this 

 epoch is often compact, clayey ; it becomes porous and friable in 

 deposites of more recent date. 



The stratified rocks must have been deposited in parallel superim- 

 posed layers, and these strata, horizontal in the beginning, have been 

 forced into the inclined and perpendicular positions which they now 

 occupy by the tumefaction or rising of the masses upon which they 

 rest. The organic remains which they present, frequently in such 

 quantity, proclaim that in the period when the revolutions of the 

 globe took place that gave them birth, there were already animated 

 beings and plants growing upon the surface of the earth. The pro- 

 duction of sedimentary strata, is an obvious proof that the igneous 

 rocks of which they are the product, must have been segregated, so 

 as to form beds of gravel, and sand, and clay. The elements of all 

 stratified rocks must necessarily have passed through these different 

 stages before the powerful causes which consolidated them, of the 

 nature of which we cannot now form an estimate, came into play. 

 The disintegration of the crystalline igneous rocks proceeds under 

 our eyes, as it were, from the combined actions of water and the at- 

 mosphere. 



Water, by reason of its fluidity, penetrates the masses of rocks 

 that are at all porous ; it filters into their fissures. If the tempera- 

 ture now fall, and the water comes to congeal, it separates by its 

 dilatation the molecules of the mineral from one another, destroys 

 their cohesion, and produces clefts which slowly reduce the hardest 

 rocks to fragments, and then to powder. During the frozen state^ 

 the ice may serve as a cement, and connect the disintegrated parti- 

 cles ; but with the thaw, the slightest force, currents of water, the 

 mere effect of weight, suffices to carry the fragments to the bottom 

 of the valley, and the rubbing and motion to which these fragments 

 of rocks are exposed in torrents, tend to break them still smaller, and 

 to reduce them to sand. 



The quantity of earthy matter brought down by streams and rivers, 

 is coripiderable : an idea may be formed of it from the thickness of 

 the slime or mud deposited by a river which has overflowed its banks 

 In many situations, the arable soil is either formed entirely, or is 

 powerfully ameliorated by such alluvial deposites. The fertilizing 

 powers of the mud of the Nile are well known ; according to Shaw, 

 the waters of this river carry with them about the 132d part of their vol- 

 ume ; those of the Rhine, at the periods of its great increase, bring down 

 more than the 100th part ; and Dr. Barrow, from observations made 

 in China, estimates at the 200th part of the volume of the mass of 

 iiuidi the mud and slime which are cariied towards the sea by tho 



