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this, the spicula, exposed upon the surface, preserve 

 all their points and angles, and are the same in form 

 as those which are found in the body of the rock, when 

 broken open, and which were never exposed to the 

 operation of any agent whatever since its formation. 

 Besides, they not only preserve their form, but even 

 their fine lustre in its pristine degree. Nay, more, 

 when perfectly crystallized, and thus exposed in the 

 surface of the rock, the terminal angles are preserved 

 as entire as at the moment when formed. 



Another proof of the durability of the hornblende 

 rock, or of hornblende which constitutes almost the en- 

 tire mass, is, that in sienite it manifests not the smallest 

 tendency to decomposition. This substance, it is well 

 known, has been employed in every department of civil 

 architecture, sculpture, and statuary, from the earliest 

 periods of time; exposed to every possible change of 

 temperature, and the operations of every active agent 

 that could be formed by a natural process, among the 

 accumulated ruins of ancient cities, yet it seems to have 

 experienced no material change. 



In the next place, the greenstone, or amphibolic 

 rocks (for they are not always green) wherever they 

 are presented to view in mass or in place, appear to 

 have preserved all their points and angles almost as 

 entire as when fresh broken, although exposed to every 

 extreme of heat and cold, that the climate in which 

 they exist may have experienced. 



With this species of rocks may also be associated 

 those of the amygdaloids, mandiesteins, or variolites, 



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