706 THE JOURNAL OF GEOLOGY. 



The relation existing between the olivine and the diallage is 

 the most interesting of the phenomena presented by the rock. 

 It has already been stated that but very few olivine-grains are in 

 direct contact with feldspar. Around nearly all are narrow rims 

 of pyroxene. At first glance these appear to be a sort of reac- 

 tion rim between the two minerals, but a more careful study of 

 the sections disposes of this assumption, for the surrounding rim 

 frequently broadens out and merges into a well defined diallage 

 plate (Fig. 3). In consequence of the occurrence of the olivine 

 and augite in the manner described sections of the rock exhibit a 



Fig. 3. Olivine partly surrounded by narrow rim of pyroxene, which is continuous 

 with large plate of same mineral. 8803. X ca. 18. 



kind of concentric structure, with the rounded olivine grains sur- 

 rounded by a zone of diallage, and imbedded in a mass of 

 plagioclase. Perhaps the most perfect exhibition of this associa- 

 tion of the three minerals is shown in the section of rock No. 1 103 

 from the Cloquet River, where the augite is in such large quantity 

 as to completely envelop the olivine (see Fig. 1). 



When the pyroxene is in smaller quantity the rim is much 

 narrower, and in many cases is in its turn separated from the 

 plagioclase by a fibrous growth between the last named mineral 

 and itself. This fibrous growth imitates in great perfection many 

 of the reaction rims described by various investigators * as exist- 



1 Tornebohm: Neues Jahrb. f. Min., etc. 1877, pp. 267 and 384. A. A. Julien : 

 ^Geology of Wisconsin, vol. 3, p. 235, PI. 22. F. Becke : Min. u. Petrog. Mitth. 1882, 



