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of this globe ? For I speak only of the rocks of our 

 mountains and those near the surface of the earth, and 

 not of those below the depth that have, as yet, been ex- 

 plored. To what extent is the process of decomposi- 

 tion carried, and what are the effects of those agents 

 upon substances the most, as well as the least, suscep- 

 tible of their operations ? 



To determine this question, it is necessary to exam- 

 ine the nature and capacity of the substances to be act- 

 ed upon, and the force or influence, which those agents 

 are capable of exercising upon these substances ; and 

 also the effects thereby produced. 



All mineral substances have the capacity of imbi- 

 bing, or rather of absorbing heat, or calorick : some in 

 a greater, and others in a less degree ; consequently, 

 their constituent particles are more or less subject to 

 expansion : by the abstraction of heat they are liable to 

 a proportionate degree of contraction. Hence sub- 

 stances of a foliated, or granular texture, when expos- 

 ed to a high or low degree of temperature, and fre- 

 quently alternated, lose, in part, their adhesive quality, 

 become weakened, and are disposed to disintegrate. 

 Thus, a hexadral prism of mica, when taken from a 

 rock, or its gangue, appears like a solid prism ; sub- 

 ject it repeatedly to the operations of heat, even of the 

 sun, and cold, and the laminae will separate sponta- 

 neously, so as to be perceptible. The same effect is 

 produced upon several of the granular substances des- 

 titute of cement : hence a gradual disintegration of the 

 part exposed 5 but the several species of rocks liable. 



