SHALE 357 



The light cellular ejections of a volcano are called scoria 

 or pumice. 



Pumice is feldspathic in constitution ; it is very porous, 

 and the fine pores lying in one direction make the rock ap- 

 pear to be fibrous. It is so light as to float on water. It is 

 much used for polishing wood, ivory, marble, metal, glass, 

 etc., and also parchment and skins. The principal localities 

 are the islands of Lipari, Ponza, Ischia, and Vulcano, in the 

 Mediterranean between Sicily and Naples. Both scoria 

 and pumice are properly the scum of a volcano. 



Volcanic ashes are the light cinders, or minute particles 

 of rock, ejected from a volcano in the course of an eruption. 



Obsidian is a volcanic glass. It resembles ordinary glass. 

 Black and smoky tints are the common colors. In Mexico, 

 it was formerly used both for mirrors, knives and razors. 

 Pitchstone is less perfectly glassy in its character, and has a 

 pitch-like luster. Otherwise it resembles obsidian. Pearl- 

 stone has a grayish color and pearly luster. Spherulite is a 

 kind of pearlstone, occurring in small globules in massive 

 pearlstone. Marekanite is a pearl-gray translucent obsidian 

 from Marekan in Kamschatka. 



ARGILLACEOUS SHALE, OR CLAY 6LATE. ARGILLITE. 



Slate is an argillaceous rock, breaking into thin laminae ; 

 shale a similar rock, with the same structure usually less 

 perfect and often more brittle ; schist includes the same va- 

 rieties of rock, but is extended also to those of a much coarser 

 laminated structure. The ordinary clay slate has the same 

 constitution as mica slate ; but the material is so fine that 

 the ingredients cannot be distinguished. The two pass into 

 one another insensibly. The colors are very various, and 

 always dull or but slightly glistening. 



Roofing slate is a fine grained argillaceous variety, com- 

 monly of a dark dull blue or bluish-black color, or some- 

 what purplish. To be a good material for roofing, it should 

 split easily into even slates, and admit of being pierced for 

 nails without fracturing. Moreover, it should not be ab- 

 sorbent of water, either by the surface or edges, which may 

 be tested by weighing, after immersion for a while in water. 

 It should also be pure from pyrites and evqfpr thing that can 

 undergo decomposition on exposure. 



Roofing slates occur in England, in Cornwall and Devon, 

 Cumberland, Westmoreland. 

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