PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 489 



Diabase.] 



ation products. The feldspar is in part at least plagioclase, although the alteration 

 has in most places gone so far as to obscure its original nature. The feldspar 

 frequently has a tendency to an idiomorphic form and the augite has an ophitic 

 relation to it. The augite is largely altered to a confused gray mass containing much 

 magnetite. The quartz is most probably secondary. 



One section shows on one side a narrow band, about one-sixteenth of an inch in 

 width, of a black opaque substance. In it are a few minute, elongated feldspars and 

 some small transparent areas which are now filled mostly with chlorite. This black 

 band undoubtedly represents part of the dense black rock which is attached to one 

 side of the darker hand sample. This dense black rock is evidently later than the 

 diabase porphyryte to which it is attached, and the narrow band in this slide is a small 

 mass of the latter rock, which was chilled very rapidly and solidified at the contact 

 practically as a glass, filling a crack in the other. 



A section of the red hand sample is similar to that just described. The feldspar, 

 both of the phenocrysts and of the groundmass, is much reddened and the whole rock 

 is more highly altered than the first mentioned sample. No fresh augite is present, 

 and epidote is common. There is in the slide one apparently pseudo-amygdaloidal 

 area filled largely with epidote and a clear, transparent, isotropic substance whose 

 exact nature was not determined. 



Three sections examined. 



Age. Cabotian. u. s. G. 



No. 683. DIABASE (with olivine ). 



Same locality as No. 681. 



Ref. Annual Report, x, page 77; Bulletin ii, page 107. 



Mrg. A medium-grained, very dark-gray diabase. 



Mic. M. E. Wadsworth's description of this rock is as follows:* 



" A gray crystalline rock containing lath-shaped plagioclase crystals and in the 

 section it is seen to be composed of lath-shaped plagioclases with a few tabular 

 feldspars forming diverging angles with each other and cutting the yellowish-brown 

 augite, which approaches diallage. in its cleavage. Some greenish altered olivine 

 pseudomorphs of serpentine, carrying tnngm'tite, occur, while the augite is in part 

 replaced by hornblende, chlorite, and viridite, which alteration products occupy much 

 of the mass of the section, whose structure otherwise is ophitic. " 



Among the alteration products is fibrous hornblende; a little biotite also 

 occurs. 



One section examined. 



Age. Cabotian. u. s. G. 



'Bulletin ii, p. 107. 



