MUSEUM OF COMPARATIVE ZOOLOGY. 239 



the unaltered olivine. The magnetite is found in extremely irregular 

 forms, while the pyrito grains often contain magnetite, and therefore 

 arise probably from its decomposition. 



[Q. 1'.] One of the NaiTow Dikes running parallel with the Trend of the 



Amygdaloid. 



Lens. A compact greenish rock containing crj'stals of feldspar. — 

 Section. Contains feldspar crystals, augite, magnetite, pyrite, and de- 

 composition products. The feldspars are kaolinized, or else decomposed 

 to white fibres, and contain considerable epidote, viridite, etc. The 

 augite occurs in irregular masses ; it is reddish and has well-marked 

 cleavage ; the decomposition to viridite, hornblende, and epidote is seen 

 to be well advanced, these substances forming along the cleavage lines. 

 The magnetite often shows the white decomposition characteristic of 

 menaccauite. The pyrite is probably derived from the magnetite. No 

 traces of olivine were seen. 



[9.] Section of Two Pieces of the Greenish vein-like or irregular Masses 

 found in the Hock. 



One of the fragments is composed of epidote, calcite, quartz, and an 

 opaque gray substance, perhaps kaolin, — mixed with the remains of 

 feldspar crystals. The other fragment, from one of the banded veins, is 

 composed of the same substances arranged in bands. Both are proba- 

 bly areas of decomposition in the rock. 



Suinmary. 



From the details given we obtain the following generalized description 

 of the amygdaloid proper. In the hand specimens the groundmass varies 

 in color from green, through gray, to red, — the last color characteristic 

 of the rock that is least decomposed. It sometimes encloses large green 

 or white feldspar crystals, often indented by the groundmass, or the feld- 

 spar crystals may be comparatively minute ; grains and crystals of epidote 

 are occasionally seen. The rock generally contains greenish spots of epi- 

 dote and of chloritic material, in part true amygdules, and spots of red- 

 dish decomposition. There are also amygdules of calcite and quartz. 



The specimens from which the eight sections were made difter chiefly 

 in the degree of decomposition, the presence or absence of olivine, and 

 the coarse or fine texture. The specimens from the eastern end are much 

 less decomposed than those from the western end. 



