192 



A PERIDOTITE FROM MAINE. 



rough quadrangular blocks of all sizes up to several feet iu diameter. 

 Everywhere the rock is firm and fresh appearing, there being no dis- 

 integration from the effects of the weather, the reddish-brown discolora- 

 tion on the surface, so far as observed, never extending to a depth of 

 over one-fourth of an inch. The stone breaks with asmooth, slightly con- 

 cave fracture, and presents to the unaided eye no crystalline secretions, 

 though greenish Hecks scattered uniformly through the mass indicate 

 the presence of serpentine, and the general appearance of the rock is 

 such as to suggest at once a peridotite, a suggestion which the micro- 

 scope fully confirms. 



The mass as a whole is remarkably uniform in color and texture. 

 1 adeed, with the except ion of an occasional small vein of serpentine mat- 

 ter not over one-half an inch in width no observable difference could be 

 found throughout the entire bill. So great uniformity iu a mass of this 

 size is rarely observed. 



As seen under the microscope the rock is composed almost wholly of 

 serpentinized olivine, augite, and scattering iron oxides. The augite 

 occurs in broad plates, with deep, rounded embayments, and in long 

 arm-like forms reaching out and enfolding the altered olivine, the pecul- 

 iar habit of the mineral in acting as a binding constituent being here 

 displayed in its best development. It is not markedly pleochroic in 

 the thin section, varying only from nearly colorless or yellowish to a 

 faint wine color. The mineral shows well-developed prismatic cleav- 

 ages and gives extinction in sections parallel to cc P do of almost ex- 

 actly 40°. With the exception of the olivine and a few small grains of 

 iron ore it is quite free from iuclosures or cavities of any kind. In 

 most cases it is beautifully fresh and unaltered; iu others it is com- 

 pletely changed. The alteration in such cases begins with a bleaching 

 and fraying out along the borders and cleavage lines, and by degrees 

 the entire mineral becomes converted into an aggregate of faintly polar- 

 izing scales and libers no longer recognizable as augite. In a few in- 

 stances direct conversion into a greenish clorite was observed, but in 

 no case does secondary hornblende or black mica appear. 



The most interesting feature of the augite is that shown in the ac- 

 companying outline sketches and somewhat indistinctly in Figs, li, 3, 



and 4 of Plate xxxiv. On casual inspection by ordinary light the 

 mineral presents no features other than of the ordinary type, the rounded 



