ITS MACROSCOPIC CEiARACTEUS. 103 



It has been a])Ove stated that tlie least altered of the terrestrial peri- 

 dotites present an appeaninee and structure essentially siuiilar to the 

 meterorite of Chassigny and the portions of that fi-oni 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 

 dark 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 yellowi.sh- or rusty-brown. The rocks 

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

 tion, a reddish-brown 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 more compact, as well as of a higher specific gravity and less 

 hardness. 



The crystalline 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 wdien the change 

 has progressed further, the groundmass becomes compact and apparently 

 homogeneous. 



As the more highly 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 green color and more 

 compact texture than the groundmass, but wliioh finally become completely 



