486 G. H. Williams — Volcanic Hocks of South Mountain 



great structure-planes of the mountain, have all been readily 

 interpreted as indications of stratification and conformity. The 

 cleavage-dip in the sandstone has often been mistaken for bed- 

 ding, while the thin jointing and slaty structure of the lavas, 

 though a secondary feature, have seemed to geologists not 

 very familiar with recent volcanic rocks, sufficient proof of 

 sedimentary origin. 



In spite of great age and some alteration, however, the 

 volcanic rocks of South Mountain have preserved all the 

 essential characteristics of our recent rhyolites and basalts in 

 such perfection that the proofs of their real nature are, to the 

 student of comparative petrography, overwhelming, while to 

 all who will candidly examine them they must be at least con- 

 vincing. 



3. Petrograjjhical Character. — Approximately 175 square 

 miles of the area of South Mountain is occupied, between Mt. 

 Holly and the head of the Middleton valley, with volcanic 

 rocks. These belong to two types which exhibit sharp con- 

 trasts of color, composition and weathering. One type is 

 inclined to tints of red, pink, blue, or purple ; is acid in com- 

 position ; generally porphyritic ; and weathers into thin slabs. 

 The other type is almost invariably of a green color; basic in 

 composition ; frequently amygdaloidal ; and weathers into 

 rough, angular blocks. The rocks of the first type have been 

 called felsite, orthofelsite, porphyry, or petrosilex ; those of the 

 second, trap, greenstone, chlorite-slate, or epidote-slate. In 

 view, however, of the perfection with which these rocks have 

 preserved the most characteristic features of their modern 

 equivalents, there is no reason why they should not, like them, 

 be termed rhyolites and basalts. The insignificance of mere 

 age as a factor in rock nomenclature is now so fully recognized 

 that we may with propriety employ the names of our recent 

 lavas for rocks of any geological horizon, when we can prove 

 beyond doubt their identity. 



It is, of course, to be expected that many rocks of inter- 

 mediate character will be found within this volcanic area. 

 Since the petrographical and chemical study has, however, 

 only begun, and since the contrast above noted is so well 

 defined, the distinction of two types may at present be regarded 

 as sufficient. 



a). The acid rocks, rhyolites. — The rocks of the acid type 

 occupy somewhat more than two-thirds of the volcanic area of 

 South Mountain — (see map). They occur in dykes and flows 

 forming a body of great thickness; they are accompanied by 

 ashes, tuffs and breccias ; they are usually porphyritic, though 

 not always so ; they exhibit the characteristics of recent glassy 

 and half-glassy rocks — flow-structures, perlitic structure, litho- 



