76 ON GEOLOGY. 



tated in rain, and forms torrents by which it returns with various degrees oi 

 rapidity into the common reservoir. This restless motion and progress of the 

 water in the form of rain or torrents gradually attenuate and wear away the 

 hardest rocks, and carry their detached parts to distances more or less con- 

 siderable ; whence we meet with limestone, clay, quartz, or flint, sand, and 

 mineral ores, in places to which they do not naturally belong. The influence 

 of the air, and the varying temperature of the atmosphere, facilitate the atte- 

 nuation and destruction of these rocks. Heat acts upon their surface, and 

 renders it more accessible, and more penetrable to the moisture, as it enters 

 into their texture ; the limestone rocks are reduced by efflorescence, and the 

 air itself affords the acid principle by which the efflorescence is continued. 

 Such are a few of the numerous causes that contribute to the disunion of 

 concrete bodies, and powerfully co-operate with that wonderful fluid which 

 alternately forms and unforms ; which creates, decomposes, and regenerates 

 all nature. 



The immediate effects of water in the shape of rain is to depress the moun- 

 tains. But the materials which compose them must resist in proportion to 

 their hardness ; and hence we ought not to be surprised at meeting occasion- 

 ally with peaks which have stood firm amid the wreck of ages, and still re- 

 main to attest the original level of the mountain-breadths which have disap- 

 peared. These primitive rocks, alike inaccessible to the assault of time and 

 to that of the once animated beings which cover the less elevated heights 

 with their relics, may be considered as the origin of streams and rivers. 

 The water which falls on their summits flows down in torrents by their late- 

 ral surfaces. In its course it wears away the soil upon which it is inces- 

 santly acting. It hollows out channels of a depth proportioned to its rapidity, 

 its quantity, and the hardness of the rock over which it passes, and at the 

 same time carries along with it fragments of such stones as it loosens in its 

 progress. 



These stones, rolled by the water, strike together, and mutually break off 

 their projecting angles ; and hence we obtain collections of rounded flints 

 which line the beds of rivers, and of smaller pebbles which the sea is perpe- 

 tually throwing upon the shores, often incrusted with a gravelly or calcareous 

 edging. The powder which is produced by the rounding of the flints, or is 

 washed down from the mountains, frequently stagnates, forms a paste, and 

 agglutinates into fresh masses of the rocky matter of which it consists ; often 

 imbedding flints and other materials, and constituting compound substances 

 known by the name of pudding-stones and grit-stones, which chiefly differ 

 from each other in the coarseness or fineness of their grains, or in the cement 

 which connects them. And if the water be loaded, as it often is, with mi- 

 nutely-divided particles of quartz, it will proceed to crystallize whenever it 

 becomes quiescent ; and will form stalactites, agates, cornelians, rock-crys- 

 tals, plain or coloured, according as it is destitute of, or combined with, any 

 colouring material: and if the material with which the water be impregnated 

 be lime instead of quartz, the crystallization will be calcareous alabaster, or 

 marble. 



Many of the earths are now known to be metallic oxides, and all of them 

 are suspected to be so : and hence a degree of heat capable of fusing 

 them, and depriving them of the ^xygen which gives them their oxide form, 

 will necessarily convert them into their metallic state. That such currents' 

 of heat, from electricity and other causes, are occasionally, and perhaps in 

 different places perpetually, existing beneath the surface of the earth, the 

 Neptunian is as ready to admit as the Plutonic geologist ; and hence the ori- 

 gin of metallic minerals, of mines, ores, ochres, and pyrites. 



The decomposition of animal and vegetable matter contributes largely, 

 moreover, in the view of the system now before us, to the changes which the 

 globe is perpetually sustaining. The exuviae of shell and coral animals is 

 pepetually adding to the mass of its earths, and laying a foundation for new 

 islands and numerous beds of limestone, in which we very often perceive 

 impressions of the shells from which the soil has originated. On the other 



