276 S: Powers — Notes on Hawaiian Petrology. 



rounded olivine nodule one inch by half an inch in diame- 

 ter was found at Kapulena, northeastern Hawaii, near 

 "Waipio Gulch in the lavas from Manna Kea. 



The origin of the xenoliths raises a number of questions 

 which have been discussed for many years, 27 but which 

 cannot be treated here. It is difficult to conceive of the 

 formation of the nodules from pre-existing rocks as no 

 rock masses of similar composition are known in the 

 Hawaiian Islands. Therefore it appears that they are 

 cognate xenoliths formed in the early crystallization of 

 the magma in depth under such conditions that olivine and 

 diallage crystals of comparatively large size and uniform 

 grain were formed. 



Recent investigations of artificial melts and of field evi- 

 dence concerning crystallization and gravitative differ- 

 entiation have shown the early formation and frequent 

 segregation and settling of olivine and the later resorp- 

 tion of olivine in the normal course of crystallization. 28 

 It has been held by Bowen that the formation of mono- 

 mineralic rocks is possible only in the earliest stages of 

 slow crystallization and that in this process "there must 

 be a considerable period during which olivine is separat- 

 ing alone and also a considerable period during which 

 pyroxene and plagioclase, although separating together 

 for the most part, form only a small fraction of the 

 total mass (liquid and crystal)/ ' 29 If sudden eruption 

 took place, segregations of olivine in the magma at this 

 period of crystallization would be carried to the surface 

 in the same manner in which inclusions are carried up- 

 ward in dikes 30 and the xenoliths of uniform composition 

 and rounded to subangular form would be frozen in a rock 

 of quite different composition. Resorption would be 

 limited in« amount owing to the lowering of the tempera- 



27 A bibliography of papers on the origin of olivine nodules is given by 

 F. Zirkel, trber Urausscheidungen in Kheinischen Basalten, Abh. math.-phys. 

 Kl. k. sachischen Ges. Wiss., 28, no. 3, Leipzig, 1903. Also see E. O. Hovey, 

 Trans. Amer. Acad. Nat. Hist., vol. 13, 1894; C. H. Kichardson, Science, Oct. 

 22, 1897; and H. Rosenbusch, Mikroskopische Physiographie, vol. 2, p. 1195. 



28 Bowen and Anderson, The binary system MgO-Si 2 , this Journal, vol. 37, 

 1914; O. Anderson, The system anorthite-forsterite-silica, ibid., vol. 39, 

 1915; N. L. Bowen, The ternary system: diopside-forsterite-silica, ibid., vol. 

 38, 1914; N. L. Bowen, Crystallization -differentiation in silicate liquids, ibid., 

 vol. 39, 1915; Powers and Lane, Magmatic differentiation in effusive rocks, 

 Trans. Amer. Inst. Min. Eng., vol. 54, pp. 442-57, 1916. 



29 The later evolution of the igneous rocks, Jour. Geol., vol. 23, Supple- 

 ment, p. 79, 1915. 



30 S. Powers : The origin of the inclusions in dikes, Jour. Geol., vol. 23, 

 pp. 1-15, 166-182, 1915. 



