APORHYOLITE OF SOUTH MOUNTAIN. 395 



ing the structure with the aporhyolites, both being portions of a single flow, led 

 to the assignment of a like origin for the structure in both types of the South 

 Mountain acid volcanics. 



Comparison with other Aporhyolites 



Harker, in a discussion of similar ancient lavas of the Bala series, describes a 

 structure which seems to be the micropoikilitic and whose origin he considers a 

 primary one. In order that the structure may be recognized, it will be necessary 

 to quote from him at some length. He says : 



" In ordinary light the groundmass (808, Craig-Cwm-Silyn I) has a micro felsitic appearance, but 

 is closely studded with round or elliptic light spots about 0.02 inch in diameter, each marked out 

 b^- a dust}' looking border. O11 using polarized light it is seen that within these little areas the 

 quartz and feldspar are more or less completely individualized, the former being in excess of the 

 latter, and the whole of the quartz in one of these spots behaves as one crystal. When the struct- 

 ure is imperfectly developed, the spots are of rather irregular form, but in the best specimens 

 (808) the} 7 are sharply defined and elliptic in outline, while the opaque looking dust, as if elimi- 

 nated from the interior, forms a narrow, elliptic ring just within the margin of the spot. The 

 structure just described seems from appearance to be an original one. It has no connection with 

 spherulitic growth, but seems to be rather of the nature of an ophitic structure, the quartz enclos- 

 ing in part the feldspathic constituent. This is well seen when, as is sometimes the case, the 

 feldspar occurs in the form of minute microlites with a partial fluxion arrangement."* 



The examination of thin-sections of specimens from the above locality reveals 

 the character of the crystallization which Harker has thus accurately described. 

 The structure which he discusses is the micropoikilitic of recent nomenclature. 

 The oval spots, with a zonal arrangement of the border, formed of globulites of 

 iron oxide and epidote, vividly recall the completely altered spherulites of some 

 of the South Mountain aporhyolites. In the extreme stage of alteration (static) of 

 an aporhyolite the spherulites are only indicated by oval areas outlined by iron 

 oxide. The true character of the oval areas is only revealed by a study of inter- 

 mediate stages of altered spherulites both in the thin-section and in the hand speci- 

 men. In the light of a comparative study of aporhyolites an adequate explanation 

 of the " elliptic spots " seems to be that there was present in them originally a 

 spherulitic crystallization. This radiating structure has been replaced, as in so 

 many cases in the aporhyolites, by the micropoikilitic structure. If this explana- 

 tion be correct, here again the micropoikilitic structure is not an " original " but a 

 secondary one. Further examination of thin-sections shows that the acid mem- 

 bers of the Bala volcanic series have every right to the appellation aporhyolite. 

 The " contemporaneous veins" mentioned by Harker seem to be altered chain 

 spherulites. Perlitic parting is conspicuous in ordinary light and is effaced in po- 

 larized light by granular crystallization. The same yielding of spherulitic centers 

 to solution and the secondary deposition of silica noted in the South Mountain 

 aporhyolites is observable in the Bala acid volcanics. 



Near Caledonia Springs, in the South mountain, occurs a conspicuously porphy- 

 rinic aporhyolite crowded with minute (one millimeter in diameter) red spherulites. 



Aporhyolites showing chain spherulites in an unusual state of preservation have 

 also been recently found in the region of Graeffenburg, Adams county. The spheru- 

 litic bands are from one to three centimeters wide and number sometimes ten to a 

 foot. The resemblance to banding in obsidians due to the same cause is striking. 



* Alfred Harker: The Bala Volcanic Series of Caernarvonshire and Associated Rocks. 1890, pp. 

 22, 23. 



