fcOME XI. GRANITE AND GRANITIC PORPHYRY. 183 



rock, they are formed by kinds of knots, or 

 large kernels of a globular figure; the sub- 

 stances appear, as it were, nodular, and disposed 

 in concentric layers ; it seems that they might 

 be produced by a small whirling motion in the 

 fluid where the rock has coagulated*; and they 

 resemble those knots which are seen in alabaster, 

 and other rocks produced by concretions, when 

 the water which deposited them .was agitated. 

 Posterior granites are most often deprived of 

 grains of pure quartz, or display smaller, and in 

 less quantity. The argil predominates more in 

 the whole mass ; and the felspar does not appear 

 in it of exactly the same nature, since it admits 

 a larger portion of calcareous earth, which per- 

 haps is not at all essential to the composition of 

 the first. 



" More than three-fourths of the antique gra- 

 nites of the monuments of Rome, are deprived 

 of grains of quartz; among others, the beautiful 

 reddish granite called Rosato, of which such im- 

 mense columns and so many Egyptian monu- 

 ments have been formed ; and in which I have 

 discovered a considerable number of small octae- 

 dral crystals of opake yellow jacinth. Often in 

 these granites, mishapen crystals, or grains of 

 ' 



* Owing perhaps to gases ? 



