THE DISTRIBUTION OF ANCIENT VOLCANIC ROCKS. 1 3 



arranged either irregularly, or in more or less discon- 

 tinuous bands or layers ; 



4. a flow structure, produced either by the elongation of 

 vesicles or the parallel arrangement of constituents 

 or crystallites. It may also be produced by the 

 interlacing of different colored magmas (eutaxitic 

 structure) ; 



5. corroded phenocrysts, quartz with embayments, or 

 skeleton crystals due to rapid and imperfect growth ; 



6. microscopic spherulites, globulites, trichites, crystal- 

 lites, real or devitrified glass inclusions, quartz with 

 orientated siliceous aureoles, axiolites, etc.; 



7. perlitic structure, wholly or partly devitrified. 

 Although some of these structures may occasionally occur in 



dykes or other igneous rocks which have rapidly solidified beneath 

 the surface, they are nevertheless so essentially characteristic of 

 effusive lavas, that, in lack of any evidence to the contrary, they 

 may be regarded as fairly safe guides in establishing the effusive 

 nature of rocks. This evidence is beyond doubt, if such rocks 

 are accompanied, as they generally are, by ash material. 



While a single one of these characteristics may not be suffi- 

 cient to identify a volcanic occurrence, many, if not all of them, 

 will be found to occur together, and only in rare instances will 

 it be found that some of them, at least, have not survived the 

 vicissitudes of metamorphism. That many regions in the ancient 

 crystalline belt of the Appalachian system exhibit most of them 

 in great perfection is now well known. It is only a misinterpre- 

 tation of these characteristic features of volcanic rocks, due to 

 a lack of acquaintance on the part of observers with their recent 

 analogues, that has prevented their recognition long ago. Thus, 

 by those who have heretofore described these rocks as sedi- 

 ments, both secondary cleavage, and the banding due to flow or 

 parallel spherulitic layers have been mistaken for stratification ; 

 spherulites have been erroneously regarded as concretions ; and 

 the accompanying pyroclastics, as normal conglomerates or 

 slates. 



