86 Evolution of the Earth. 



the stream ? Large rocks and boulders, perhaps, from 

 which the finer material has been washed away ; pebbles 

 and coarse gravel, but no fine mud. The finer sediment has 

 been carried on to the lower country whither the stream is 

 making its way ; and there, if you observe, you Avill find 

 that the bottom of the stream is covered first with fine sand, 

 and farther on, as it wends its way slowly through the level 

 country, with layers of mud or clay. Thus, everywhere 

 along the course of mountain torrents, in brooks and rivers, 

 and inland lakes and in the mighty ocean itself, has gone 

 on the process of differentiation, breaking up the primeval 

 rock, distributing its particles, and preparing for the later 

 stages of the transformation. This action of rivers and 

 water-courses was probably somewhat augmented at the close 

 of each glacial period, when the vast masses of ice were 

 melting, flooding the valleys with water, changing their 

 form by the force of the torrents, and everywhere deposit- 

 ing new layers of detritus. 



Let us now observe what goes on in this subsequent part 

 of the process, — how the work of integration is effected. 

 We cannot see what takes place at the bottom of the ocean, 

 but we can judge of it by observing similar processes along 

 the banks of our lakes and rivers. At certain periods they 

 become dry — the sediment at the bottom is exposed. Or, 

 we can watch what occurs where pools of water have been 

 formed in depressions of the soil by a heavy rain, after the 

 water evaporates. We can cut through a layer of the de- 

 posited sediment and observe what it contains. The upper 

 portion of it will exhibit the later deposits of silt thrown 

 down as the water was ebbing away, mingled with dead 

 leaves and sticks and whatever may have been caught in 

 the eddies of the stream and held fast as it subsided ; the 

 lower part, the coarser gravel. If consolidated into rock, 

 this gravel forms a loosely compacted conglomerate, or 

 "pudding-stone " ; the finer sand the different varie- 

 ties of sand-stone, according to the material of which 

 it is composed, and the still finer mud and silt, some 

 kind of shale or slate. Much of the material of which 

 sandstone is made is deposited in shallow water along 

 the shores of the sea. When in its original state, 

 before it is dried and solidified, it readily receives the 

 impress of whatever is brought in contact with it. If 

 a second layer of sand is blown into the impressions 



