DESCRIPTIONS OF IiOCKS. 



415 



the rest of the material in a comparatively fine-grained con- 

 dition. The porphyry of the ancients was a rock of dark 

 feldspathic base, sprinkled all through with light-colored 

 feldspar crystals ; and, from this fact, any metamorphic or 

 igneous rock containing such disseminated crystals of a 

 feldspar is said to be porpJiyritic. 



The following figures illustrate three varieties of porphy- 

 ritic rock. The first represents a specimen of the red an- 

 tique porphyry of Egypt— now often called Rosso antico — 

 the rock which gave the name porphyry to geology, a kind 



Antic 



Oriental Verd-antique. Porphyritic gneiss. 



much used by the Eomaris (though not by the Greeks or 

 Egyptians), and quarried by them in the mountain Djebel- 

 Dokhan, twenty-five miles from the Eed Sea, in latitude 

 27° 20'. Through the red aphanitic base small whitish 

 crystals of orthoclase are thickly distributed. Figure 2 is 

 from a polished piece of green antique poryphyry. The 

 feldspar crystals are comparatively large, and the compact 

 base has a dark green color. Figure 3 represents a large 

 crystal of orthoclase with the gneiss about it, from porphy- 

 ritic gneiss. The feldspar crystals in porphyritic gneiss or 

 granite sometimes measure three inches by one and a half, 

 and again only a fraction of an inch. These orthoclase 

 crystals, as often in other porphyritic rocks, are twin crys- 

 tals, the plane of cleavage of one half making an angle of 

 52° 23' with that of the other half. Occasionally large crys- 



