chap. xiii. Porphyritic Rocks of. 437 



resemble a common sandstone. These beds are sometimes 

 marked with true lines of aqueous deposition, separat- 

 ing particles of different degrees of coarseness ; in other 

 cases there are parallel ferruginous lines not of true 

 deposition, as shown by the arrangement of the par- 

 ticles, though singularly resembling them. The more 

 indurated varieties often include many small and some 

 larger angular cavities, which appear due to the re- 

 moval of earthy matter : some varieties contain mica. 

 All these earthy and generally white stones insensibly 

 pass into more indurated sonorous varieties, breaking 

 with a conchoidal fracture, yet of small specific gravity ; 

 many of these latter varieties assume a pale purple tint, 

 being singularly banded and veined with different shades, 

 and often become plainly porphyritic with crystals of 

 feldspar. The formation of these crystals could be 

 most clearly traced by minute angular and often par- 

 tially hollow patches of earthy matter, first assuming a 

 fibrous structure, then passing into opaque imperfectly 

 shaped crystals, and lastly, into perfect glassy crystals. 

 When these crystals have appeared, and when the 

 basis has become compact, the rock in many places 

 could not be distinguished from a true clay stone por- 

 phyry without a trace of mechanical structure. 



In some parts, these earthy or tufaceous beds pass 

 into jaspery and into beautifully mottled and banded 

 porcelain rocks, which break into splinters, translucent 

 at their edges, hard enough to scratch glass, and fusible 

 into white transparent beads : grains of quartz included 

 in the porcelainous varieties can be seen melting into 

 the surrounding paste. In other parts, the earthy or 

 tufaceous beds either insensibly pass into, or alternate 

 with, breccias composed of large and small fragments 

 of various purplish porphyries, with the matrix gene- 

 rally porphyritic : these breccias, though their sub- 



