80 



OUTLINES OF GEOLOGY. 



186. Next in order we may cite the mechanical action of 

 water, by which portions of the more elevated parts of the 

 surface of the land are continually torn off, or worn away, 

 and again deposited in the forms of boulders, pebbles, gravel, 

 sand, and mud. In the deltas of great rivers we find these 

 occupying large extents of what was formerly a part of the 

 ocean, and arranged in regular layers. In such formations 

 we find imbedded the remains of animal and vegetable life. 

 The enormous quantity of solid matter continually carried 

 into the ocean by rivers, which far exceeds the alluvium 

 which they add to the extent of the continents, shows us that 

 similar formations must be taking place in the depths of the 

 sea ; these must contain all the matter found in deltas, modi- 

 fied by the mechanical action of the sea itself, and charged 

 in addition with the remains of the animals which inhabit the 

 ocean, or bound and aggregated by the labours of the coral- 

 line zoophytes. 



187. The surface waters of the earth not only exert a 

 mechanical, but also a chemical action. The most extensive 

 formations of this character are composed of carbonate of 

 lime, taken up by water holding an excess of acid, and de- 

 posited when that acid escapes. The sulphate of lime, which 

 is soluble without any intermediate agent, is also deposited in 

 the form of carbonate, receiving the necessary acid from the 

 atmosphere. Such calcareous deposits take the form of sta- 

 lactite, travertine, and tufa ; they also convert beds of peb- 

 bles, gravel, sand, and clay, into conglomerates, calcareous 

 sandstones, and marl. Silica is also carried in solution by 

 water, and deposited ; carbonate of iron is dissolved by ex- 

 cess of acid, and thrown down in the form of hydrated oxide 

 generating beds of bog ore, or applied in cementing deposits 

 of mechanical origin. 



In the gradual decay of organized structures, these chemi- 

 cal deposits take the place of the decomposing substances, 



