600 PROCEEDINGS OF BOSTON MEETING. 



perceptible quantity of the acmite molecule. The extinction angles in the prism 

 zone are as high as 40°. Marked zonal structure is characteristic of many crystals 

 representing a difference in extinction angle sometimes as great as 8°. An analysis 

 of this mineral is printed under V in the preceding table. The one and a half per 

 cent of sodium present ex})lains the yellowish green absorption of the mineral. 

 The extensive resorption which has occurred in the case of many individuals is of 

 considerable interest. This does not see?m to have taken place with any uniformitj'^ 

 about the crystal, but has frequently made its way to the center of the crystal from 

 one side, even when the greater part of the surface remains unattacked. The ma- 

 terial thus dissolved has again crystallized in place as an aggregate of nearly color- 

 less pyroxene, magnetite and plagioclase. Such resorption has frequently gone so 

 far as to produce perfect magmatic pseudomorphs after the augite phenocrysts. In 

 the examination of the hand specimen these can be seen as dark colored areas with 

 good crystal boundaries, having the appearance of basic inclusions. Under the 

 microscope the outlines are easily identified as those of augite, since octagonal basal 

 sections are found having the pyroxene angle. Moreover, remnants of the original 

 augite crystals, which are sometimes found in these pseudomorphs, indicate by 

 their orientation and cleavage that they once filled the entire space of the inclusion. 

 The pseudomorphs are easily distinguished from the host by the large amount of 

 magnetite, the needles of colorless pyroxene, and the base of plagioclase of which 

 they are composed. The pyroxene of the normal rock is, as already mentioned, 

 green, and in the groundmass it plays quite a subordinate role. Unlike the rock 

 groundmass, the pseudomorphs contain no vesicles and no glassy base. They must 

 be explained in the same manner as are the magnetite-augite crowns about biotite 

 and hornblende in the andesites. While the lava stood in the pipe of the voh^ano 

 a differentiation of the magma occurred, which rendered the already formed augite 

 phenocrysts unstable. Their resorption began in consequence, and owing to the 

 great viscosity of the lava the diffusion of the resorbed material was practically nil. 

 As a consequence the material recrystallized in situ, but on account of the chanced 

 conditions, in other chemical molecules. These augite pseudocrystals show analo- 

 gies with the leucite pseudocrystals, which have been described from Arkansas'^ 

 and Brazil.f 



A mineral which was occasionally observed in the rock, having a high index of 

 refraction, a high double refraction, and a rough surface, was identified as olivine, 

 though its presence was hardly to be expected. 



The groundmass of the rock has a glassy base, in which are contained a great 

 number of microlites of felspar and a much smaller number of small augite crys- 

 tals. It contains occasional larger or smaller irregular shaped inclusions, which 

 appear to be partially fused fragments of dolerite. A rare white inclusion which 

 was found later by Iddings on his visit to the island and kindly given me to ex- 

 amine is undoubtedly a metamorphosed liparite. It is composed chiefly of felspar 

 and quartz. Cleavage pieces of the felspar show the optical characters of 

 ''normal" sanidine. A partial analysis of the inclusion has been made for me by 

 Mr Leo C. Urban with the following results : 



SiOa 74.57. AlA 13.09. CaO 1.32. 



* J. F. Williams : The Igneous Rocks of Arkansas. Ann. Kept. Geol. Surv. Ark., 1890, vol. ii. p. 2C7. 

 fEugeu Hussak : Neues Jahrb., 1892, vol. ii, p. 159. 



