14 ALTEENATIONS. [Ch. H, 



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

 and impure coal, alternating with shales and sandstones, and underneath 

 the whole, perhaps, are calcareous strata, or beds of limestone, filled with 

 corals and marine shells, each bed distinguishable from another by cer- 

 tain fossils, or by the abundance of particular 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, conglom- 

 erate 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, color, 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 un- 

 dermine 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 after year, and century after century ; 

 but I may suggest an explanation of the manner in which some micaceous 

 sandstones have originated, namely, those in which we see innumerable 

 thin layei-s 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 gueiss, 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 be thus understood. If we take a 

 handful of quartzose sand, mixed with mica, and throw it into a clear 

 running stream, we see the materials immediately sorted by the water, 

 the grains of quartz falling almost directly to the bottom, while the plates 

 of mica take a much longer time to reach the bottom, and are earned 

 farther down the stream. At the first instant the water is turbid, but 

 immediately after the fiat surfaces of the plates of mica are seen all alone 

 reflecting a silvery light, as they descend slowly, to form a distinct mica- 

 ceous lamina. The mica is the heavier mineral of the two ; but it re- 

 mains a longer time suspended in the fluid, owing to its greater extent of 

 surface. It is easy, therefore, to perceive that where such mud is acted 

 upon by a river or tidal current, the thin plates of mica will be carried 



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

 "Deltas," " Water," <fcc. 



