80 THE CRYSTAL FALLS IRON-BEAEING DISTRICT. 



The following t.able will show the arrangement outlined above, which 

 will be followed in the descriptions: 



Classification of the rocks of the Eemloclc formation. 



> Schistose acid 1 

 ( Nonporphyritic . 



^ T.TPfls * Ehyolite-porphyry .... ) o„i,i„j.„f,p „„;,•. i„^„„ 



Acid .. ] ^^""^^ I Aporhyolite-porphyry . \ »cnistose acm iavas . 



( Pyroclastics 



( Lavas Metabasalt -j Porphyritic 



Basic . ' / Variolitic 



i Pyroclastics-Eruptive breccia \ ^J'^'P^^I.^e'^ir^^. \\V. 



f Volcanic sediments. .. . \ ^""^'^ ^«P°"*« j Ash^beds :".;:; 



„ ,. , I ( Subaqueous deposits.. .Conglomerates 



becumentary . ■. (Slate , 



Normal sediments Li,ii6stone 



Crystalline 

 schists. 



ACID VOLCAlSriCS. 



The acid volcanics are comparatively unimportant in quantity. They 

 may be conveniently subdivided into the lavas and pyroclastics. 



ACID LAVAS. 



The acid lavas occur in such small quantity as to make it impossible 

 without very great exaggeration to place them iipon the accompanying 

 small-scale general maps, though they have been introduced upon the 

 detail maps wherever the scale permitted. They usually foiTn isolated 

 ridges, and their relations to the surrounding basic volcanics are obscured 

 l)y lack of exposures. The trend of the individual ridges agrees with the 

 general strike of the banding in the basic tuffs. Moreover, in nearly all 

 cases the isolated exposui-es which are closest together lie in such relations 

 to one another that when connected the large sheets thus formed follow the 

 strike of the tuff banding, as do the individual ridges, and they are there- 

 fore confidently assumed to be the isolated portions of acid flows inter- 

 bedded with the basic volcanic rocks. 



The rock types represented are the two closely related rocks — the 

 rhyolite-porphyry and the aporhyolite-porphyry. Under the rhyolite- 

 porphyries are included the porphyritic acid lavas, which have, so far as 

 can be determined, an original holocrystalline groundmass. Under the 

 aporhyolite-porphyry, following Miss Bascom's use of ffj;o,^ I include those 



' structures, origin, and nomenclature of the acid volcanic rocks of South Mountain, Penn- 

 sylvania, by Miss Florence Bascom : Jour. Geol., Vol. 1, 1893, p. 816. 



