part 4] NOEiTE or sierra leone. 327' 



relative proportions of lime and magnesia present in the magma. i 

 Yogt has shown elsewhere " that, when magnesia is largely in excess 

 of lime, the rhombic pyroxene would crystallize first ; it would 

 then be followed by intergrowths of mix -crystals, and these latter- 

 by monoclinic pyroxene alone, in accordance with lloozeboom's. 

 Type TV of mix-crystals. There occurs also a lamellar inter- 

 growth of hy^Dcrsthene and augite similar to that of the pyroxenes, 

 in the St. David's, Whin Sill, and Duluth rocks ; it ranges from 

 strong banding down to lamellation so thin as to be scarcely 'per- 

 ceptible microscopically. The finer phases of the lamellation and 

 other intimately-related forms are probably due to deposition from 

 solid solution. With reference to similar phenomena in the- 

 labradorite-norite of the Lofoten Islands (Norwa}^), Prof. J. H. L. 

 Yogt has stated that ^ 



'By cooling down to ordinary temperature, ... the enstatite-hypersthene 

 cannot carry so much Ca(Mg, Fe)Si.,0,. in solid solution. Consequently some 

 augite is separated ou.t in perthitic lamellae — corresponding with the perthitic- 

 lamellge of albite or albite-oligoclase in perthitic striated microcline.' 



The relations of the pyroxenes to the other j)rincipal minerals, 

 depend upon the relative propoi'tions of the various minerals in the 

 magma : when the p^a'oxenes are greatly in excess of the general 

 eutectic, they are the first minerals to crystallize out. The eutectic 

 minerals enter into an interesting series of intergrowths. 



The pale augite occurring as rounded inclusions in the diallage 

 is probably the monoclinic form of magnesia-pyroxene which Dr. 

 Elsden has briefly discussed in his St. David's paper.'* 



Olivine. — This mineral is a common and often important con-- 

 stituent of both the older and the younger norites ; some varieties 

 of the norite consist almost entirely of olivine and felspar, while 

 certain bands in the banded norite are very rich in coarse granular 

 olivine. Olivine is absent from the normal beerbachite ; but it 

 is frequently present in this rock as xenocrysts derived from the 

 norite. Olivine occurs principally in the norite, both as rounded 

 blebs and as more or less idiomorphic crystals ; sometimes it 

 occurs as strings and clusters of contiguous crystals which, while 

 presenting rounded outlines to other minerals, are themselves 

 separated by clean crystal-faces. The cr3^stallization of the olivine 

 extended over a considerable period : where the mineral was rela- 

 tively abundant it formed fairly idiomorphic crystals, but where it 

 was in relatively small proportion, on the other hand, it developed as. 

 skeleton-crystals enclosing pyroxenes and laths of felspar. Locally, 

 however, it formed a beautiful poecilitic struclure with the pyro- 

 xene ; sometimes it entered into intergrowth with other minerals, 

 (see below). 



1 'Die Silikatschmelzlosungen ' pt. i (1903) p. 129 & pt. ii (1904) p. 109,. 

 Vidensk. Selsk. Skrifter, Christiania. 



2 See J. V. Elsden, ' Principles of Chemical Geology ' 1910, p. 166. 



•' 'On Labradorite - Norite with Porphyritic Labradorite- Crystals, &c.'' 

 Q. J. C. S. voL Ixv (1909) p. 101. 

 4 Ibid. vol. Ixiv (1908) p. 288. 



