Pogue, Jr. — Geology and Structure of Volcanic Hocks. 225 



The coarse tuff is mostly rhyolitic to dacitic in character. 

 Some occurrences contain a large number of andesitic frag- 

 ments ; and in this way a transition is made into the andesitic 

 tuffs and breccias. 



Acid volcanic breccia. — The acid volcanic breccia is practi- 

 cally confined to one band, about one-half mile in width, which 

 extends through the central portion of the district. Asso- 

 ciated with the breccia are found outcrops of the acid tuffs, 

 flows of rhyolite and andesite, and long narrow strips of ande- 

 sitic tuffs and breccias. The acid breccia is twofold in char- 

 acter, and comprises both a brecciated phase of rhyolite, and 

 a very coarse tuff, with fragments predominant over ground- 

 mass and larger in size than one-half inch. The rock is locally 

 called "mountain rock"; its outcrops are large in size and 

 extremely abundant. Enormous bowlders up to 20 and 30 

 feet in diameter are frequent, and with larger, half buried 

 masses make up rocky ridges which are almost impassable. 

 Where most exposed, the rock becomes white and pitted upon 

 its surface. Further weathering forms a porous, sponge-like 

 exterior which is characteristic. Great concentrically weath- 

 ered plates are at every stage of peeling off; frequently 

 spherical shells several yards across may be pried off with a 

 small pick. 



When freshly broken, the breccia has a mottled grayish color. 

 A great number of light colored, angular fragments make up 

 most of the surface. Groundmass and broken feldspar pheno- 

 crysts fill in between the fragments. Irregular masses of dark 

 green material, present in some phases of the rock, are seen 

 on close inspection to represent andesitic fragments; these are 

 never very abundant, In places, phenocrysts and fragments 

 have an alignment suggestive of fluw structure. Most of the 

 outcrops are massive. 



By a gradual decrease in number and size of fragments, the 

 breccia passes imperceptibly into the rhyolite : and in almost 

 any part of the formation isolated outcrops of the rhyolite 

 may occur surrounded by the breccia. Part of the breccia, 

 consequently, is considered a phase of rhyolite brecciated 

 through flowage. Much of the breccia, however, is probably 

 an ordinary air breccia. It is impossible to separately delimit 

 the two phases in the field. 



Rhyolite. — Rhyolite occurs in narrow, elongated areas, 

 associated with the acid volcanic breccia, into which it grades. 

 It is found best developed along the crest of Flat Swamp 

 Mountain. The occurrences represent the remains of old lava 

 flows. 



The rhyolite forms prominent, rounded outcrops ; and is 

 predominantly massive and somewhat jointed. Its surface is 



