ITS MACROSCOPIC CHARACTERS. 163 



It has been above stated that the least altered of the terrestrial peri- 

 dotites present an appearance and structure essentially similar to the 

 meterorite of Chassigny and the portions of that from Estherville which 

 are comparatively free from iron. They are of a grayish-green or green 

 color, crystalline granular in structure, and usually contain more or less 

 dai'k grains of picotite or some iron ore, disseminated throughout the mass. 

 The first traces of change are in coloration, passing from a green to a 

 yellowish-green, yellowish, and to a yellowish- or rusty-brown. The rocks 

 are more or less vitreous or greasy in lustre. With increasing altera- 

 tion, a reddish-bi'own to grayish-brown color predominates ; and this 

 finally passes into a dark greenish-black to black compact rock, somewhat 

 resembling the basalts, but of a duller color, more resinous lustre, 

 and nioi"e compact, as well as of a higher specific gravity and less 

 hardness. 



The crj'stalline granular groundmass of olivine or serpentine may or may 

 not porphyritically enclose crystals and grains of enstatite, diallage, and 

 augite. These minerals usually appear as greenish, grayish, or bronze-like 

 crystals and grains scattered in the rock. They commonly weather to 

 bronze-like, more or less cleavable and platy forms ; and even on the fresh 

 fracture of some specimens show in certain lights as an irregular network, 

 brightly reflecting the light, and holding in its meshes the dark-greenish 

 altered olivine. 



The olivine groundmass when altered presents under the lens the appear- 

 ance of yellowish or grayish granules cut and surrounded by a fine reticu- 

 lated network of a darker material (serpentine) ; but when the change 

 has progressed further, the groundmass becomes compact and apparently 

 homo2:eneous. 



As the more hitrhlv altered states are reached, the variations in the 

 macroscopic appearance become exceedingly numerous ; so much so, that 

 only a few of them can be mentioned here. The color generally is some 

 shade of green, varying from a dark green or greenish-black to a yellowish- 

 green. Sometimes it is reddish (brownish- or cherry-red), owing to the state 

 of its ferruginous contents. The pyroxene minerals, when occurring, vary 

 in amount of alteration from the porphyritically sprinkled bronze- or copper- 

 like crystals to silvery-white, and to grayish and greenish forms, which in 

 turn pass into patches of serpentine of a deeper gi-een color and more 

 compact texture tlian the groundmass, but which finally become completely 



