in Pennsylvania and Maryland. 487 



physae, spherulites (in masses, in layers and in chains), axiolites, 

 pumice, amygdaloids, etc., etc. — in hardly less perfection than 

 the specimens which Professor Iddings has so admirably 

 described from the Yellowstone Park. 



These structures are preserved, in spite of the recrystalliza- 

 tion of the entire rock substance into a fine mosaic. They are 

 therefore most apparent in hand specimens, especially when 

 brought out by weathering ; or, under the microscope, they 

 are bettor seen in ordinary than in polarized light, as was the 

 case with the old glass breccia, recently described by the writer 

 from the Sudbury region.* 



The following is an analysis of a rhyolite specimen from 

 the Gladhills road, near the Bigham Copper mine, on the 

 north side of Pine Mountain, made by Mr. C. Hanford Hen- 

 derson of Philadelphia, and published in 1884.-f- This is a 

 quite typical rhyolite analysis. When compared with analyses 

 of our most recent acid lavas, the iron may seem a little high 

 and the alumina a little low, but on the whole the close agree- 

 ment is a surprise. 



SiO„ 73-62 



Al 2 6 3 12-22 



Fe„0 3 2-08 



FeO 4-03 



CaO 0-34 



MgO 0-26 



Na„0 3-57 



K 2 2-57 



Ign. 0-40 



Total 99-09 



The macroscopic features of the rhyolite are the best proofs 

 of its true nature, for weathering brings out on the surface of 

 the rock each delicate detail. In this way we discover every 

 characteristic of glassy rocks, though there is no glass now 

 remaining. Plate X, fig. 1, shows in photographic reproduc- 

 tion a specimen five inches long, covered with lithophysse as 

 perfect as any the Yellowstone can furnish. Their delicate 

 petals are a pale pink, while the base has weathered white. 

 With a lens the radiating and minutely fluted, concentric struc- 

 ture can be traced distinctly. Fig. 4 (p. 488), gives a some- 

 what diagrammatic idea of this structure. 



Plate X, fig. 2, shows the delicate lines of flow-structure as 

 they are displayed on the weathered surface of a specimen which 



*Bull. Geol. Soc. Am, vol. ii, p. 138, 1891. Ann. Kept. Geol Surv. Can. for 

 1889-90, F, p. 75, 1891. 



f "The Copper Deposits of South Mountain," by C. H. Henderson. Trans. Am. 

 Inst. Mining Engineers, vol. xii, p. 90. 



