phalen] the rocks of nugsuaks peninsula 1 95 



Peridotite var. picrite 



At first glance this peridotite (Cat. No. 75,487) might be mis- 

 taken for a basalt, with an interlacing network of segregated 

 olivine. It is black in spots, or rather has a blackish-brown augitic 

 base, through which the olivine runs in zigzag courses, producing a 

 mottled or inlaid appearance, remarkably clear and striking. A 

 vein-like penetration of olivine into the black ground mass was also 

 observed. On closer examination the grain of the rock proves to 

 be rather fine, the olivine granules, however, being macroscopically 

 distinct, while those of the ground mass are practically irresolvable. 



Under the microscope the following minerals were observed — oli- 

 vine, augite, chlorite, feldspar, biotite, magnetite, limonite, hematite, 

 and apatite. Of these constituents olivine is by far the most 

 abundant. It occurs in perfectly automorphic forms, excepting 

 when corroded by the surrounding magma. The crystals are 

 in large part isolated and distinct from each other. Often very 

 irregular masses are present. When isolated, there is, as a general 

 rule, a development of the micropoecilitic structure, though in no 

 case are the isolated particles broken from a parent crystal, but 

 are per se crystalline units. In many instances the olivines present 

 a completely shattered aspect. Fissures penetrate them in all direc- 

 tions, often arranged radially with respect to the center and generally 

 filled with a light-brown ferritic pigment. This phenomenon is not 

 attributable to any pressure which the rock has sustained, for no 

 evidences of strain are visible in any of the sections studied. Only 

 in a single instance was undulatory extinction noted. Not only are 

 the individual phenocrysts shattered, but the entire sections them- 

 selves seem to be rifted in a more or less regular manner. Along 

 these zones of fracture the rock is broken up into a series of parallel 

 cracks, filled as with the olivines with brown and black iron oxides. 

 We have here a phenomenon closely related to rifting, a microscopic 

 phase of jointing, as it were. 



In many instances the olivines are in an advanced state of altera- 

 tion and often the entire nucleal portions of crystals have been con- 

 verted into light green, slightly dichroic prochlorite. This chlo- 

 ritization furnishes the key to the explanation of the radiating cracks 

 observed in the olivine, for the process is essentially one of hydra- 

 tion and expansion, which produces the radial cracks observed.^ 

 The process is not always nucleal, however, for simultaneous with 

 and independent of these changes in the interior of the crystal, 



^ See plate liv, 3, in the automorphic olivine, represented in the middle of 

 the illustration. 



