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XV, On Porphyry, 

 By Professor Jameson. 

 (Head Utk Becemhei' 181 l.J 



According to Werner, there are five diftereitt 

 kinds of porphyry, viz. clay-porphyry, hornstone- 

 porphyry, pitchstone-porphyry, obsidian-porphy- 

 ry, and pearlstone-porphyry ; all of which occur in 

 Primitive country, and either in a conformable or 

 unconformable and overlying position He does 

 not enumerate in his system of rocks any Transi- 

 tion porphyry ; and in the Floetz class, the only 

 rock that can be considered as appertaining to this 

 species, according to the Wernerian view, is his 

 Porphyritic- stone, or Porphyritic claystone. 



* I may here remark, that it is not improbable that some 

 porphyries described as overlying, are not so in reality, being 

 merely conformable beds cut into variously-shaped masses by the 

 action of the weather ; and these, when viewed in particular di« 

 sections, appear to be uncomform^le and overlying. 



