516 W. S. Bayley — Fibrous Intergrowth of 



eastern Minnesota,* the attention of the writer was repeatedly 

 attracted by a fibrous growth around olivine, that resembles 

 very strongly the reaction rims that have been described as 

 existing between garnet and serpentine by Schrauff and 

 Becke,:}: between garnet and olivine by Diller,§ and between 

 the last named mineral and feldspar by TornebohmJ Julien,^" 

 Becke,** G. H. Williams,ft Schuster,^ Teal! ,§§ and Lacroixff 

 A close study of the phenomenon, however, disclosed the fact 

 that the growth is not due to reactionary processes between 

 the rock's constituents, but is simply an original intergrowth 

 of two of them. Its appearance, nevertheless, is so suggestive 

 of a reactionary origin (see fig. 1), that it has been thought 

 well to describe it briefly. 



The rock in which the phenomena occur is a very coarse 

 grained gabbro, composed of a perfectly fresh olivine with 

 the composition of hyalosiderite, a pink or purple diallage and 

 labradorite, whose analysis yielded Dr. Hillebrand the follow- 

 ing figures : 



Si0 2 



Alo0 3 



Fe 2 3 



FeO 



CaO 



MgO 



K 2 ]S T a 2 



H 2 Total 



51-89 



29-68 



0-32 



0-37 



12-62 



0-38 



0-50 3-87 



0-46 = 100-09 



Its structure is typically gabbroitic, in that the plagioclase is 

 the youngest of the three components, and not the jjyroxe?ie T 

 as in the case of the diabases. "f^f It is peculiar, however, in the 

 fact that in most sections the diallage, instead of occurring in 

 grains and plates scattered indiscriminately among the other 

 constituents, exhibits a fondness for the proximity of the oli- 

 vine. As illustrated in figure 2 (specimen from the Falls of 

 the Cloquet Kiver, SEJ Sec. 84, T. 53 JST., E. 14 W.) the 

 diallage envelops the olivine grains and so separates them 

 from the surrounding plagioclase It is evident that the first 

 mineral to form from the magma was olivine. Then followed 

 the pyroxene, which attached itself to the surfaces of the 



* R. D. Irving: Copper-Bearing Rocks of Lake Superior: Mon. v, U. S. G. S., 

 p. 266; this Journal, III, vol. xxxrv, 1887, p. 204 and 249; and 7th Ann. Rept. 

 TJ. S. a. S.. 1888, p. 419. 



f Neues Jahrb. I Mm., etc., 1884, ii, p. 21, and Zeitschr. f. Kryst., vi, p. 321. 



% Min. u. Petr. Mitth., iv, pp. 189, 285, 323-6. 



§ This Journal, xxxii, 1886, p. 122. 



|| Neues Jahrb. f. Min.. etc., 1877, pp. 267 and 384. 



I Geology of Wisconsin, III, p. 235 and Plate XXII. 

 **Min. u. Petrog Mitth., 1882, iv, pp. 330, 350, 450. 

 f+ Bull. U. S. Geol. Survey. No. 28, p. 52. 



X X Neues Jahrb. f. Min., etc.. B. B. v, p. 451. 

 |§Min. Magazine, Oct., 1888, p. 116. 



II Bull. Soc. Franc d. Min , 1889, xii, p. 83. 



"f[^[ So far as the knowledge of the writer extends the rock of this flow is the 

 only true gabbro in northeastern Minnesota. The other coarse-grained pyroxene- 

 plagioclase rocks of this region, described by Irving and others under the name 

 of gabbro, are either diabases or diabase-porphyrites. 



