222 DOMAIN n. SILICEOUS. 



STRUCTURE II. FELSITE PORPHYRY, 



Saussure mentions a porphyry with a base of 

 earthy felspar. What is called klingstein por- 

 phyry, or porphyry slate, by Werner, is the most 

 common and at the same time the most remark- 

 able substance in this division. It has been al- 

 ready described under the Mode Felsite, 



STRUCTURE III. PITCH-STONE PORPHYRY. 



This has been chiefly observed in Auvergne, 

 where the base is generally a dark or bottle-green 

 pitch-stone, with lighter crystals of felspar. It 

 also occurs in the island of Arran. 



In those parts of Auvergne which are truly 

 volcanic (a position to which the most rigid dis- 

 ciples of Werner, who have visited that region, 

 such as Buch and Daubuisson, among others, 

 have been converted), pitch-stone is often found 

 decomposed, and partly reduced to a brownish 

 mass, resembling ochre of iron, and probably 

 arising from the five parts of iron which it con- 

 tains. This substance will be more minutely de- 

 scribed in the division of Decomposed Rocks. 



