520 W. S. jBayley — Fibrous Intergrowth, etc. 



so complete (Figs. 2, 4, 5 and 1), that the inference is justified 

 that the highly refractive component of this intergrowth is 

 diallage. In the coarse-grained gabbros the mantle around the 

 olivine is certainly diallage ; in those cases where a very thin 

 seam is interposed between the fibrous growth and the mineral 

 it envelops, the material of this seam is as surely a pyroxene. 

 Moreover, the augite of the rock frequently sends out prolon- 

 gations of its own substance into the surrounding plagioclase, 

 and the ends of these prolongations have characteristics that 

 are identical with those of the more highly refractive compo- 

 nent of the intergrowths. Finally, in the coarser rims, their 

 constituents may be traced on the one side into plagioclase and 

 on the other side into diallage. 



We may safely infer, therefore, that the fibrous intergrowth, 

 which so closely resembles a reaction rim between olivine and 

 plagioclase, but which is surely not such, is merely a grano- 

 phyric aggregate of plagioclase and pyroxene. The major 

 portion of the latter mineral in the rock separated before the 

 feldspar, and taking advantage of the surfaces afforded by 

 the already formed olivine and magnetite, fastened upon them. 

 But in many cases, before the crystallization of the pyroxene 

 had ceased, the feldspar began to form, and the two minerals 

 crystallized together micropegmatitically. 



So far as known to the writer, only one other description of 

 a similar intergrowth of augite and plagioclase in a manner 

 resembling a reaction rim is to be found in petro graphical 

 literature. In 1887 Camerlander* mentioned the existence of 

 such an intergrowth around garnets in a contact rock from near 

 Prachatitz in the Bohemian Forest. The original description 

 has not been seen, but in a review of it by Becke,t words are 

 nsed that might well be applied to the phenomenon observed 

 in the Minnesota rock: "Especially are the garnets often 

 surrounded by the micropegmatitic intergrowth of augite and 

 plagioclase. This varies remarkably in its appearance from 

 that of undoubted augite imbedded in feldspar, to that of a 

 dense radial bundle of fibres which resembles the kelyphite 

 rims around garnets in serpentine." 



United States Geological Survey, 

 Lake Superior Division. 



*Jahrb. d. K. K. geol. Reichsanst, xxxvii, p. 117. 



f '• Besonders die Granaten sind haufig durch solche rnikropegrnatitische Ver- 

 wachsungen von Augit und Plagioklas umrandet. Dieselbe variire ausserordent- 

 lich in ihrer Ausbddungvon deutlichen in Feldspath eingewachsenen Augits biszu 

 radial gestelten dichter Faserbuscheln, welch an die Kelypkitranden der Pyrope 

 von Serpentin erinneren." Kef. Neues Jahrb. f. Min., etc., 1888, ii, p. 54. 



