PART I. CHAPTER II. 29 



Alternations Sorting Power of Water. 



sists of one of the above rocks, sometimes of two or more in 

 alternating beds. Thus, in the coal districts of England, for 

 example, we often pass through several beds of sandstone, some 

 of finer, others of coarser grain, some white, others of a dark 

 colour, and below these, layers of shale and sandstone or beds 

 of shale, divisible into leaf-like- laminse, and containing beautiful 

 impressions of plants. Then again we meet with beds of pure 

 and impure coal, alternating with shales, and underneath the 

 whole, perhaps, are calcareous strata, or beds of limestone, 

 filled with corals and marine shells, each bed distinguishable 

 from another by certain fossils, or by the abundance of particu- 

 lar species of shells or zoophytes. 



This alternation of different kinds of rock produces the most 

 distinct stratification ; and we often find beds of limestone and 

 marl, conglomerate and sandstone, sand and clay, recurring 

 again and again, in nearly regular order, throughout a series of 

 many hundred strata. The causes which may produce these 

 phenomena are various, and have been fully discussed in my 

 treatise on the modern changes of the earth's surface.* It is 

 there seen that rivers flowing into lakes and seas are charged 

 with sediment, varying in quantity, composition, colour, and 

 grain, according to the seasons ; the waters are sometimes flooded 

 and rapid, at other periods low and feeble ; different tributaries, 

 also, draining peculiar countries and soils, and therefore charged 

 with peculiar sediment, are swollen at distinct periods. It was 

 also shown that the waves of the sea and currents undermine the 

 cliffs during wintry storms, and sweep away the materials into 

 the deep, after which ■ a season of tranquillity succeeds, when 

 nothing but the finest mud is spread by the movements of the 

 ocean over the same submarine area. 



It is not the object of the present work to give a description of 

 these operations, repeated as they are, year afler year, and cen- 

 tury after century ; but I may suggest an explanation of the 

 manner in which some micaceous sandstones have originated, 

 those in which we see innumerable thin layers of mica dividing 

 layers of fine quartzose sand. I observed the same arrangement 

 of materials in recent mud deposited in the estuary of La Roche 

 St. Bernard in Brittany, at the mouth of the Loire. The sur- 

 rounding rocks are of gneiss, which, by its waste, supplies the 

 mud : when this dries at low water, it is found to consist of brown 

 laminated clay, divided by thin seams of mica. The separation 

 of the mica, in this case, or in that of micaceous sandstones, may 



* Consult Index to Principles of Geology. " Stratification," " Currents," 

 Deltas," " Water," &c. 



