PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 195 



Apobsidian.] 



the line crossing it. It is necessary to conclude that these grains are derived from 

 a single quartz crystal which has been broken and the fragments slightly dislodged 

 from each other. 



lhe/1'tt/npars, seen in the thin sections, have a distinct cleavage, but no twin- 

 ning. They are generally charged with ferruginous and other impurities. There 

 are no glassy feldspars visible in the sections made, and they are very rare, or entirely 

 wanting in the hand sample. There are, however, a few clea^able feldspars, visible 

 macroscropically, w r hich are quite different from the flesh-colored feldspar pheno- 

 crysts, but they are hardly distinguishable from the quartzes. None of these appear 

 in the sections examined. 



The matrix, as now altered, between crossed nicols becomes very dark, but in 

 some parts of the slide it is permeated by quartz which has been formed in the pro- 

 cess of devitrification. This quartz is sometimes in interlocking areas, each having 

 its own orientation, clouded only by the secondary microliths, and occasionally it is 

 gathered in what may have been fissures or other cavities, and is clear and pure. 

 The orientation of this clear quartz, however, governs the extinctions in that of the 

 adjoining matrix, and it is plainly of the same date and origin. The porphyritic 

 quartzes, in a similar manner, govern the quartz orientation in the adjoining matrix, 

 but they as plainly preceded the poikilitic quartz in the matrix. The contact 

 of these quartzes with the colored matrix is sharp, while the matrix gradually 

 thins out about the poikilitic quartz, some of its substance sometimes being isolated 

 in specks or spots in a dwindling manner within the quartz. It may be assumed 

 safely that the poikilitic borders of the porphyritic quartz are of the same date 

 as the poikilitic quartz of the body of the rock. 



Three sections; one preparation. 



A i/i'. Cabotian; red-rock series. 



liciHin-h. This rock is one of the quartz porphyries of Irving, but as it has 

 changed from a surface and glassy condition, as evidenced by an occasional 

 vesicular structure visible in the hand samples and indicated by its general rhyolitic 

 nature, it is more accurately described as a rhyolyte, to which the prefix <ip<> is 

 attached as suggested by Miss Bascom, to correspond with its altered present 

 condition. N. H. w. 



No. 130. APOBSIDIAN. 



A short distance northeast of the mouth of Beaver creek, lying below No. 129, suddenly thrust upward. 

 Jief. Annual Report, ix, page 33. 



The pieces at hand afford two kinds of rock, one a gray, cherty-looking 

 very fine-grained rock, apparently embraced in the other, which is also brownish 

 to gray, but somewhat coarser and apparently belongs with the Two Harbor series 



