GEOLOGICAL SOQIETY OF PENNSYLVANIA. 303 



the mountains of the Blue Ridge, and, like all other en- 

 tritic groups, they have a decided crystalline structure, 

 and are of a heterogeneous composition, which varies fre- 

 quently, as the different constituents individually predo- 

 minate. They penetrate and pass into each other, some- 

 times by slow gradations, leaving no distinct line of 

 demarcation between two rocks materially different. 

 Again, the passage is abrupt between rocks widely 

 differing in physical appearance, whatever may be the 

 analogy of composition. There is a want of continuance 

 or permanence of mineral composition, and consequently 

 of specific character. We shall endeavour to convey an 

 idea of the whole, by special descriptions of some of the 

 most prodominant rocks, and their relative positions. 



VARIOLITES ARGILO-PORPHYRIES. 



In ascending the ridge from the west, a variety of grey 

 schistose rocks and shales are crossed before arriving at 

 a rock, which at a little distance would be taken for a 

 conglomerate. On close examination, it will be found 

 that the spheroidal masses that are embedded in the base 

 consist of petro-silicious crystals, sometimes white, some- 

 times green. These globular masses are generally semi- 

 crystaliine. Again, the cavities are only partially filled, 

 and the substances coating the interior have a more per- 

 fect crystalline form. 



The spheroidal masses embedded, and the crystals 

 coating the sides of the cavities, differ in colour and 

 composition from the base of the rock, which is gene- 

 rally bluish-brown, reddish-grey and green, varying in 

 intensity from time to time. The base of this amygda- 

 loid has the appearance of being an altered argillaceous 

 schist, and is by far the most abundant part of the rock. 

 Rolled masses are found which, though not as beautiful, 

 are not dissimilar to the rolled variolitcs that are found 

 in the Durance, departnicnt of Drome, in France. 



