ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN. 821 



scale by Irving, 1 is essentially nothing else than a special phase 

 of the fluidal structure, a phase peculiar to flowage in lava consol- 

 idating with extreme rapidity, that is, in an acid glass. The 

 granular crystallization has developed with entire disregard to 

 these curved patches, shreds and stringers. 



LitlwpJiysal structure. — Often the macroscopic features of the 

 South Mountain acid volcanics disclose their original character 

 more convincingly than does the microscope. Lithophysae are 

 one of the structures which are best revealed in the hand- 

 specimen, where they are brought out in delicate relief by 

 weathering. The rose-pink petals of the lithophysae in a paler 

 pink base produce quite as beautiful specimens of this glassy 

 structure as any rhyolite shows. The ?nicro-pegmatitic structure 

 shows itself in microscopic pegmatoid groups of phenocrysts 

 such as are found in the Yellowstone rhyolites. 2 



Perlitic parting. — That this structure is occasionally present in 

 the South Mountain rocks in great perfection has already been 

 noted. While its presence is a most reliable test of the former 

 character of the rock, its absence furnishes no evidence against 

 the previous glassy condition of the rock, both because many 

 recent rhyolites show no trace of that structure and because it is 

 most readily effaced by devitrification. 



Amygdaloidal structure. — In some localities the acid volcanics 

 are conspicuously amygdaloidal. The bright green amygdules 

 of epidote in a pale pink matrix render this rock strikingly 

 handsome. In a few instances 3 the vesicles, which, as seen 

 under the microscope, are bordered by a broad rim, like the 

 ground-mass in crystallization, but are separated from it by a 

 clear zone of silica and are darkened by an abundance of black 

 iron oxide, bear on the inner edge of this border spherulitic 

 growths. These are surrounded by a clear zone of- silica while 

 the center of the vesicle is filled either with an opaque black 



"Irving: opus cit., pp. 312-313, Fig. 22. 



2 Iddings : opus cit., p. 275, PI. XV., Fig. 5. 



3 In specimens from Racoon Creek at the east base of Piney Mountain, south of 

 Caledonia Furnace. 



