Augite and Plagioclase in a Minnesota GaUbro. 517 



olivine grains and continued to grow until it had abstracted all 

 pyroxenic material from the still liquid mass. The residue 

 solidified as labradorite. When the diallage is present in the 

 rock in large amounts it completely envelopes the olivine ; 

 when it is present in small quantity only, it forms a very nar- 

 row rim that may or may not surround the older mineral. 

 Although the association of olivine and diallage is not as de- 

 scribed in all sections, it is sufficiently common in the rock to 

 emphasize the tendency of the latter to arrange itself around 

 the former. 



Fig. 1. Olivine and biotite sur- 

 rounded by fibrous growths resem- 

 bling reaction rims. Section 7025. 

 x30. 



Fig. 2. Section of olivine-gabbro, 

 exhibiting the tendency of the py- 

 roxene to include olivine grains. 

 Section 1103. x 20. 



In one-third of the sections of this rock studied, the olivine 

 grains, where they would otherwise come in contact with 

 plagioclase, are kept from doing so by a finely fibrous growth 

 that polarizes in some places with bright colors, and in others 

 with the bluish-gray tint of thin feldspar. Though in general 

 these fibers extend perpendicularly from the bounding surfaces 

 of the olivines, they occasionally form radial groups centering 

 at points near the peripheries of the surrounded mineral. Nor 

 are the fibers in all cases continuous. They often branch and 

 fork, and frequently stop suddenly at short distances from 

 their points of origin, while new fibers begin their courses 

 just beyond. This fibrous envelope, moreover, is not confined 

 exclusively to the vicinity of olivine. It is found also around 

 decomposed biotite (see upper left hand corner of lig. 1) and 

 around grains of magnetite (fig. 3). 



A close inspection of the sections often reveals the presence 

 of an intermediate zone between the fibrous one and the min- 



