MODE XII. SLATE. 



leagues. The difficulty of supplying such phe- 

 nomena has led some to imagine a plastic force 

 in nature, a power of modelling, in the mineral 

 kingdoms forms analogous to those of organised 

 bodies. 



" These slates also often present beautiful py- 

 rites in the form of trees, more than a foot in 

 extent, which are regarded by Guettard as im- 

 pressions of tremellte. The pyrites is sometimes 

 in small grains, disseminated like a dust upon 

 the surface of the slates ; where may also be ob- 

 served many little stars of selenite. 



" When the blocks have been drawn from the 

 quarry, if they be left exposed to the sun or to 

 the open air for some days, they lose what is 

 called the quarry-water, become hard and un- 

 tractable, so that they can only be employed in 

 building. Frost produces a singular effect on 

 these blocks : while frozen they may be divided 

 with more ease than before; but if thawed a 

 little quickly, they are no longer divisible. Yet 

 this quality may be restored by exposing them 

 once more to frost; but if the alternative be 

 often repeated, it becomes impossible to reduce 

 them to leaves. 



" The secondary slate which is found in other 

 countries, offers nearly the same dispositions and 



