No. 3.] 



MACPARLANE— ON CRYSTALLINE ROCKS. 



277 



of 64.34 p. c. The homogeneous, nearly infusible matrix of the 

 same rock contains 67.36 p. c. of silica. The labradorite por- 

 phyrite of Miihleuthal, near Elbingerode in the Hartz, possesses 

 a black, very fresh and hard matrix, which encloses undecom- 

 posed very lustrous crystals of labradorite, and a dark green or 

 black augitic or hornblendic mineral. The labradorite contains 

 51.11 p. c. silica, while the whole rock, in spite of the presence 

 of the, doubtless more basic, black mineral, contains 57.57 p. c. 

 silica. On the other hand, in many porphyries and rhyolites 

 distinct quartz crystals are developed, which, of course, must be 

 more acid than the enclosing matrix. In spite of this exception, 

 the law above referred to still applies so far as regards the 

 minerals developed in crystalline rocks or separated out from 

 their matrices during solidification, 



VIII. — SPECIFIC GRAVITY, 



It has been already remarked that in general the specific 

 gravity of original rocks decreases with the increase of silica and 

 increases with the decrease in quantity of the same substance ; 

 the most acid rocks are specifically the lightest, the most basic 

 rocks are specifically the heaviest. Abich was the first to call 

 attention to this as exhibited among the volcanic rocks, and to 

 shew the conclusion^ which might be drawn regarding the silica 

 contents of these rocks from their ascertained specific gravities. 

 Although the same relation has been observed to exist among 

 the granitic and porphyritic rocks, and doubtless runs through 

 all the orders, it has not been found that a certain specific gravity 

 invariably corresponds to a certain degree of silicification or that, 

 for instance, because a syenite containins; 59.83 p. c. of silica has 

 a specific gravity of 2,730, a trachyte having the same silica 

 contents will have the same specific gravity. On the contrary 

 we find decided differences as to specific gravity in rocks of 

 similar composition, but belonging to different orders of texture. 

 The following table shews the average specific gravity of the 

 various families of granular, porphyritic and trachytic rocks : 



