522 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



around them, nor any unusual abundance of the ordinary 

 minerals of the essexite. Hence we may assume that the 

 portions of shale assimilated merely augmented the felspars 

 of the magma and perhaps to some extent the augite, by 

 supplying hypersthene molecules produced by the inter- 

 action of quartz with olivine. A few rounded crystals of 

 quartz have been found in the pallio-essexite, which are 

 undoubtedly grains of sand derived from arenaceous bands 

 in the shales. One of these which occurs in a section of 

 specimen L is reproduced in fig. 1, Plate XXXIX. The 

 quartz is all one crystal, but it has been much cracked, 

 and chlorite has forced its way into the cracks. It is 

 surrounded by a reaction rim of a very pale green or color- 

 less fibrous mineral disposed in an irregularly radial manner, 

 which may be either actinolite or simply a non-titaniferous 

 augite. It is evidently formed by reaction of the quartz 

 with the olivine molecules still in the magma, with the 

 inclusion perhaps of some anorthite and wollastonite mole- 

 cules. The amphibole or pyroxene of this reaction rim is 

 now a good deal chloritized. 



Indirect evidence of assimilation of the shales is perhaps 

 to be found in the more acid character of the plagioclase 

 of the outer envelope as compared with the rock of the 

 main mass. Microscopic evidence alone suggested this 

 comparative acidity, as the maximum extinction angles in 

 symmetrical sections of the plagioclase crystals were found 

 to be lower in specimens coming from near the contact 

 than in the specimens from the main mass. Subsequent 

 calculation from analyses of the average compositions of 

 the plagioclase of the typical specimens amply confirmed 

 this view. In specimen B of the pallio-essexite the aver- 

 age compositions of the plagioclase was Ab 7 An 3 , in speci- 

 men I of the main rock it was Ab 4 An 3 . That the more 

 acid character of the felspars of the outer envelope is due 

 to assimilation, and not to some kind of differentiation 





