THE BASIC MASSIVE ROCKS, ETC. 707 



ing between olivine and plagioclase in many basic rocks. It 

 usually consists of very fine fibres extending perpendicularly 

 from the bounding surfaces of the diallage rim, or when this is 

 lacking - , from the peripheries of the olivine grains. In a few in- 

 stances the fibres form radial groups, centering at points on the 

 exterior of the surrounded mineral. The growth is especially 

 noticeable in the vicinity of the olivine, but it is occasionally 

 also found bordering magnetite grains (Fig. 4) and flakes of 

 biotite. The fact that the fibres are not confined to the borders 



Fig. 4. Fibrous intergrowth around magnetite (?) Between the latter mineral and the 

 fibrous rim can be seen a narrow zone of diallage. Section 10439. X 20. 



of olivine, but are found as well around magnetite, biotite, 2 and 

 outside of the diallage rims around olivine grains, is presumptive 

 evidence that the growth is not of reactionary origin. 



Between crossed nicols portions of the fibrous zone polarize 

 brilliantly, while other portions have the pale blue tint of thin 

 feldspar. Under very high powers the individual fibres are dis- 

 covered to be discontinuous. They branch, fork and bend in a 

 fantastic manner, and sometimes stop abruptly, while new fibres 

 begin their courses some distance beyond and continue to the edge 



iv., pp. 330, 350, 450. G. H. Williams : Bull. U. S. Geol. Survey, No. 28, p. 52. M. 

 Schuster : Neues Jahrb. f. Min. etc., B. B. v. p. 451. Teall : Mineralogical Maga- 

 zine, Oct. 1888, p. 116. Lacroix : Bull. Soc. France d. Min., 1889, xii., p. 83. 



2 The biotite is probably secondary so that the occurrence of the fibrous rim 

 around it is of little importance as an aid in determining its nature. 



