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and, although sometimes found in situations highly 

 elevated, do not contribute so abundantly to the forma- 

 tion of alluvial districts, nor, by decomposition even, to 

 the increase of the soil which they often overhang. 

 With the latter I am ready to admit, that from its in- 

 compact and pulverulent texture, it is not only liable 

 to be decomposed, but is rendered unfit, except in some 

 instances, for the purposes of civil architecture. Yet 

 however liable it may be to decay, it forms but a small 

 integrant part in the great mass of rocks, and common- 

 ly lies low and is mostly covered ; consequently con- 

 tributes but little either in the formation of soil or al- 

 luvion. 



Having briefly considered some of the rocks most 

 liable to decomposition, it is necessary to take a slight 

 view of such as are least subject to disintegration. Of 

 these may be considered, 



Granite, Primitive trap, 



Gneiss, Do silicious slate, 



Sienite, Secondary lime stone, 



Compact mica slate, Green stone, 

 Clay slate (argillite) Porphyries, 

 Primitive lime stone, Quartzose rocks, &c. all 

 which are the most predominant in the great scale of 

 rocks, and most of which are employed in the various 

 departments of civil architecture, sculpture, statua- 

 ry, &c. 



Previous to entering into any remarks on the decom- 

 position of these last mentioned rocks, it may not be 

 amiss to consider what is the nature, or actual cause, 



