im 



iiocics. 



merous. In these, petrifactions first make their appearance, 

 and they usually consist of species of corals and zoophytes, 

 which do not at present exist, and are therefore supposed to 

 be extinct. Fletz rocks are disposed in flat or horizontal 

 strata. They contain abundance of petrifactions ; and these 

 much more various in their nature than those which occur 

 in the transition formations, consisting of shells, fish, and 

 plants. The alluvial formations constitute the great mass 

 of the earth's surface. They have been formed by the gra- 

 dual action of rain and^ river water upon the other forma- 

 tions. They consist of the component parts of previously 

 existing rocks, separated by the influence of air, moisture, 

 and change of temperature, and deposited in beds. Sand, 

 gravel, loam, and petrifactions of animals and vegetables, are 

 often found in this class. Volcanic formations are pseudo- 

 volcanic, or such minerals as are altered in consequence of 

 the burning of beds of coal situated in their neighbourhood ; 

 and true volcanic, or such as are actually thrown from the 

 crater of a volcano. 



The expansion of water in the pores or fissures of rocks 

 by heat, or congelation, is a physical cause of the separation 

 of their parts. The solvent power of moisture exerted upon 

 alkaline or calcareous matter, in rocks, is another cause of 

 their decomposition. Electricity, which is shown, by ex- 

 periments with the voltaic apparatus, to be a most powerful 

 agent of decomposition, seems to assist in all these changes ; 

 electrical powers being almost constantly exhibited in the 

 atmosphere. The production of a bed for vegetation is 

 effected by the decomposition of rocks. As soon as the rock 

 begins to be softened, the seeds of lichens, which are con- 

 stantly floating in the air, make it their resting-place. Their 

 generations occupy it, till a finely-divided earth is formed, 

 which becomes capable of suppof'ting mosses and heath : 

 acted upon by light and heat, these plants imbibe the 

 dew, and convert constituent pans of the air into nourish- 

 ment. Their death and decay afford food for a more per- 

 fect species of vegetable ; and, at length, a mould is formed, 

 in which even the trees of the forest can fix their roots, and 

 which is capable of rewarding the labours of the cultivator. 

 The decomposition of rocks tends to the renovation of soils, 

 as well as their cultivation. Finely-divided matter is carried 

 by rivers from the higher districts to the low countries, and 



