PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 401 



A opobsidian. ] 



Meg. On a fresh fracture the rock is ashen gray in color. It is aphanitic, 

 contains no porphyritic crystals and presents no variations in grain except a fine, 

 indistinct mottling in lighter and darker shades of ashen gray. The specimen is 

 crossed by several, narrow, distinct bands nearly parallel and about half an inch 

 apart. These bands are from one to two millimeters wide and are of a gray to 

 pinkish color. Frequently a narrow strip along the centre of a band is darker colored 

 than the rest of the band and is seen to be composed largely of quartz. Sometimes 

 these quartz bands are wavy. The material of these bands, except for the central 

 strip of quartz, is as aphanitic and structureless as the rest of the rock. 



J/Vr. The sections in ordinary light present a cloudy appearance, with small 

 transparent areas and numerous black dots of nutyiictiii', also some licnidfite, and a 

 few larger irregular areas of magnetite. Tn a few places these grains of iron ore 

 are surrounded by a narrow rim of a highly refracting substance, probably spJi/'iir, 

 thus indicating that the iron ore is, in part at least, i/menite rather than magnetite. 

 Under crossed nicols and a high power the section breaks up into an irregular 

 patchy aggregate of small poikilitic quartz areas. The quartz encloses the iron ore 

 and minute, cloudy, semi-opaque, irregular grains which as a rule show no influence 

 on polarized light, although occasionally one is seen which shows slight changes on 

 rotating the stage. The nature of this material cannot be determined, but it is 

 thought to be largely feldspathic, most probably orthoclase. The quartz frequently 

 has a clouded appearance and thus resembles feldspar, but no cleavage is present 

 and some of the grains which resembled feldspar the most gave uniaxial interference 

 figures. 



A section across one of the bands shows hardly anything different from the rest 

 of the rock, as the band, except for the central strip of quartz, is hardly distinguish- 

 able under the microscope. If anything is to be noticed, it seems to be a greater 

 abundance of the opaque feldspathic grains in the band. The central strip is more 

 transparent and contains very little of the feldspathic material. Three sections. 



Age. Cabotian. 



Remarks. This rock is thought to have been originally a glassy rock like an 

 obsidian, and its present condition is due to devitrification. No glass, as such, can 

 be said to be present, although it cannot be said that part of the opaque so-called 

 feldspathic substance is not still glassy. The bands which cross the rock do not 

 furnish evidence as to their original nature, but it seems probable that they represent 

 chains of spheruliths, although no trace of spheruliths is present. In the hand 

 specimen these bands are quite similar to the spherulith layers in the rocks from 

 South mountain.* u. s. G. 



*F. BASCOM . Journal of <!e<>lo t /u, vol. i, pp. 818,819, 1893; U. S. Geol. Survey, Bulletin cxxxvi, plate X. 

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