488 6r. H. Williams — Volcanic Hocks of South Mountain 



in its interior is a homogeneous, dark purple felsite. On 

 other specimens these now lines are even more sinuous. This 

 specimen was merely selected from a great variety because it 

 appeared well-fitted for reproduction. 



At some localities the rhyolite is crowded with spherulites. 

 Plate X, fig. 3, represents a large specimen found north of 

 the junction of Copper run with Tom's Creek. Here the 

 spherulites make most of the mass and have no regular arrange- 

 ment. They have the size of large peas. In other cases the 

 spherulites are of smaller size and more sparsely distributed. 

 They are then not infrequently arranged in layers as described 

 by Iddings in the Yellowstone obsidian.* One specimen from 

 Raccoon Creek (No. 143) shows small but very perfect, grey 

 spherulites distributed, singly, in layers or in large aggregates 

 through a black base which was once probably obsidian. In 

 appearance it is not unlike the well known specimens from the 

 Lipari Islands. 



5. 



Fig. 4. Structure of a lithophysa brought out by weathering — natural size — 

 No. 129, from Raccoon Creek. 



Fig. 5. Hand specimen showing chains of spherulites — No. 78, from near Snowy 

 Mountain. 



Another very common arrangement of spherulites is shown 

 in fig. 5. Here they occur in single layers, which, in 

 cross-section, appear like chains, bent more or less out of their 

 normal horizontal position by the flow-motion of the magma. 

 The individuality of these spherulites is preserved, though they 

 are so merged into one another that they approach the axiolitic 

 structure. This structure is so frequent that it has doubtless 

 helped confirm the idea that the felsite and porphyries were 



* Obsidian Cliff, 7th Ann. Rept. U. S. Geol. Surv., p. 276, Plate xviii, 1888. 



