PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 497 



Gabbro.] 



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

 "Under the microscope it is seen to be composed principally of pl<i</iucl<iw, Ulal- 

 /(/</( and lif<niif<-r<inx magnetite. A little unstriated feldspar was observed. The dial- 

 lage is of a brownish color and somewhat altered, in places passing into viridite. 

 A very little biotite of secondary origin was seen in association with the diallage and 

 magnetite. The feldspar is somewhat altered to the common grayish-white product 

 of its change (kaolin?), but still retains its plagioclastic character, strongly marked 

 by its broad polarization bands." 



One section examined. 



Aye. Cabotian. r. s. G. 



No. 697. GABBRO (with olivine). 



Same locality as \o. (J95. Embraced as nodules in No. 695. 



Ref. Annual Report, x, page 81; Bulletin ii, page 90; Bulletin vi, page 13G. 



A dark-gray rock of medium grain, somewhat rusty in appearance. It is 

 composed of feldspar and much black material, part of which is biotite. 



.!/>. M. E. Wadsworth's description of this rock is as follows:f 



"The sections of this rock are composed ot olivine, diallage, feldspar, magnetite 

 and secondary serpentine, hornblende, biotite, chlorite, etc. In its general appearance it 

 is closely allied to wherlyte and picryte, but although its feldspar is subordinate to the 

 other minerals yet it contains sufficient to carry it under gabbro. The olivine, in 

 part, is clear, fissured, and traversed along the fissures by bordering serpentine or 

 ferruginous staining. Other olivines are nearly or entirely replaced by the greenish 

 serpentine. The diallage contains some of the black needle-like inclusions, etc., but 

 for the most part it has been changed to a brown, felty, cleavable mass, or to chloritic 

 and amphibole products. The feldspar in part is much altered. The biotite varies 

 from a deep reddish-brown to a brownish-yellow color colors to which its dichroism 

 corresponds. The biotite is largely associated with the magnetite, often surrounding 

 the grains of the latter." 



The rock varies considerably as regards alteration. Some areas of the slides 

 show all the original minerals in a comparatively fresh state, while in immediately 

 adjoining areas these minerals (except the magnetite) have been completely altered. 

 The feldspar is commonly twinned according to the albite law and its extinction 

 angles show it to be near lalirndnrltc. The augite is of later date than ijie feldspar 

 and olivine, and the latter mineral is later than at least some of the feldspar. 



Among the secondary minerals in one of the slides are stout needle-like forms 

 of an almost colorless mineral with a marked cleavage, high index of refraction and 

 a strong double refraction. The extinction is parallel or nearly so and the optic 



'"Bulletin 71, p. 70. 

 i /Inlli'liit ii, ]>. %. 

 33 



