708 THE JOURNAL OF GEOLOGY. 



of the rim. It is impossible to determine the character of the fibres 

 in the finest rims, but in those in which the structure is coarser, it is 

 learned that two components are present. One is possessed of a 

 high index of refraction, and strong double refraction, and this 

 appears to be continuous with the diallage of the narrow zones 

 interposed between the fibrous growth and the surrounded 

 olivine. The other component penetrates between the pyroxene 

 fibres, and has club-shaped ends. Occasionally the twinning 

 bars of plagioclase may be detected in it, and hence it is assumed 

 to be a triclinic feldspar. The fibrous rim is thus an intergrowth 

 of plagioclase and augite, both of which minerals are normal con- 

 stituents of the gabbro. In the fibrous rims they have evidently 

 crystallized contemporaneously, whereas in the main body of the 

 rock the main portion of the diallage preceded the plagioclase in 

 its separation from the magma. There is no necessity for 

 regarding the intergrowths as in any way connected with reac- 

 tionary processes, while there is abundant reason for believing 

 them to be due solely to the tendencv of simultaneously crystal- 

 lizing minerals to mutually interpenetrate each other. This 

 tendency is well recognized as existing to a marked degree 

 between quartz and orthoclase, whereby granophyre is formed, 

 and to a less extent between various other minerals. Micropeg- 

 matitic intergrowths between hornblende and feldspar, for 

 instance, have been described by Levy, 1 Camerlander 2 and La- 

 croix, 3 between hornblende and quartz by Kalkowsky, 4 between 

 garnet and feldspar by Becke, 5 and between garnet and quartz 

 by Lacroix (1. a, p. 317,) between diopside and quartz by Levy, 6 

 and between various monoclinic pyroxenes and plagioclase by 

 Becke (1. c), Camerlander (1. c), Lacroix (1. c, pp. 316 and 

 318), and Levy. 7 In the Minnesota rock the diallage in many 



'Bull. Soc. Min. d. Fr., 1878, p. 41. 



2 Ref. Neues Jahrb. f. Min., etc., 1888, II., p. 52. 



3 Bull. Soc. Franc, d. Min., 1889, XII., p. 319. 



4 Gneissformation, des Enlengebirges, p. 41. 



5 Min. u. Petrog. Mitth. 1878, p. 406. 



6 Bull, des Serv. d. 1. Carte geol. d. 1. France, No. 9, 1890, p. 7. 



7 lb. p. 7. 



