in Pennsylvania and Maryland. 489 



stratified. It seems to be quite independent of the presence or 

 absence of phenocrysts in the rock. 



Still another characteristic, which the South Mountain rhyo- 

 lites possess in common with many recent lavas, is what we 

 may call the eutaxitic structure.* This consists of the close 

 intermingling of two portions of the magma which, on account 

 of some slight differences in chemical composition or hydra- 

 tion, show a marked contrast in color. The two portions may 

 be, for instance, black and white, pink and blue, or red and gray. 

 They mingle so as to produce in some cases an irregular mott- 

 ling; in others a complex series of interlacing bands which 

 bring out the flow-structure in a striking manner. Specimen 

 No. 145, from Raccoon Creek, shows small black areas resemb- 

 ling shreds of glass, imbedded in a pink base which, on a 

 weathered surface, appears snow-white. 



Amygdaloidal structure, representing old vesicles more or 

 less elongated by motion in the viscous mass, is much less com- 

 mon in the acid, than in the basic rocks. Fine examples of it 

 were, however, found at the Bigham Copper mine ; along 

 Raccoon Creek and at the eastern end of the Jack's Mountain 

 tunnel (Nos. 20, 147 and 7). A.s a microscopic feature this 

 vesicular structure is still more common, (see fig. 6). Per- 

 litic parting, so characteristic of glassy rocks, may be detected 

 macroscopically in many felsite specimens. 



The microscopical characters of the South Mountain rhyo- 

 lites are far too varied to be described in a brief paper like the 

 present. It must suffice to say that, in spite of the recrystal- 

 lization of the substance, we still find in great perfection all 

 the essential features of the most recent acid lavas. Corroded 

 and skeleton phenocrysts of quartz, micropegmatitic inter- 

 growths of quartz in feldspar phenocrysts, f and phenocrysts 

 broken by the flow movement. In the groundmass we find 

 spherulitic tufts, axiolites, elongated vesicles filled with quartz, 

 trichites and globulites in great variety, which bring out each 

 detail of flow-structure. All the particulars of microscopic 

 structure call for extended study and description. For the 

 present purpose, however, which is merely to establish the 

 character of these rocks, what has been said must suffice. 



In mineralogical composition the South Mountain rhyolites 

 are quite uniform. When phenocrysts are present, the most 

 abundant are alkali feldspar. Quartz in rounded bipyramids 

 is always to be found with the feldspar under the microscope, 

 although it is not so often apparent to the unaided eye. In 

 only a single instance (No. 257D, from the head of Miney 



*Fritsch and Reiss: Tenerife, p. 414, 1868. Rosenbusch: Die Massigen Ge- 

 steine, 'M Ed., p. 625. 



f J. P. Iddings : loc. cit, p. 2*74 and Plate xv. 



