DESCRIPTIONS OF ROCKS. 415 
the rest of the material in a comparatiyely fine-grained con- 
dition. ‘The porphyry of the ancients was a rock of dark 
feldspathie base, sprinkled all through with lght-colored 
feldspar cr ystals ; ; and, from this fact, any metamorphic or 
igneous rock containing such disseminated crystals of a 
feldspar is said to be po) “pha yritie. 
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 
al 2. 3 
; LM, is, WA 
Has ve 

MUA Mi Hl bi 
Rosso Antico, Oriental Verd-antique. Banuigaiie gneiss. 
much used by the Romans (though not by the Greeks or 
Egyptians), and quarried by them in the mountain Djebel- 
Dokhan, twenty-five miles from the Red 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 
oranite sometimes measure three inches by one and a half, 
and again only a fraction of an inch. These orthoclase 
erystals, 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- 
