124 PETROLOGY OF THE ALKALINE ROCKS 



The rock is therefore a liniburgitic basalt almost identical with kulaite. 



The pseudomorphs of this rock, by the presence in them of felspar, strongly suggest 

 that the molecular alteration of the original hornblende was accompanied by replace- 

 ment of its molecules by molecules of the ground-mass. 



P. 324 (648). Kulaite, Cape Bird. — This rock is similar to J. 36, but the alteration 

 of the hornblende has not progressed so far. 



The texture of this rock is hypocrystalline and porphyritic, with phenocrysts, 

 partly idiomorphic and partly broken and corroded, scattered about as in vitro phyric 

 texture. 



The phenocrysts consist of felspar belonging to the labradorite and andesine families, 

 faintly pleochroic augite (light green to light brown), unaltered barkevicitic hornblende 

 in idiomorphic crystals, and pseudomorphosed basalt hornblende in corroded crystals. 

 Apatite occurs as inclusions in all the minerals, and magnetite is very abundant as a 

 secondary constituent and both primary and secondary in the base. 



The pseudomorphs consist of a granular mixture, chiefly magnetite, with colourless 

 augite and red hypersthene in some cases : in other cases the main alteration product 

 is a red hypersthene phenocryst with a rim of secondary magnetite and augite. It is 

 possible that this hypersthene is an original mineral, but both the association and the 

 occurrence in it at times of traces of hornblendic cleavage suggest that it is secondary. 



Some of the felspar phenocrysts have tuning-fork ends. 



The base consists of idiomorphic felspar laths, magnetite titaniferous augite, and 

 an isotropic glass. The magnetite of the base is largely secondary. 



P. 325 (648). (For Analysis see Table IV, B, p. 120 ; and Table V, No. VIII, p. 122. ) 

 This rock slice is similar to the foregoing. It is markedly porphyritic in faintly pleo- 

 chroic augite and basalt hornblende, undergoing the alteration described previously. 

 Some pseudomorphs consist mainly of a large crystal of red hypersthene with a mag- 

 netite-augite rim. Others consist mainly of an aggregate in which magnetite pre- 

 dominates over augite and hypersthene. The original phenocrysts of hornblende were 

 highly idiomorphic, but they have been largely broken and corroded in their meta- 

 morphosis. This idiomorphism is noticeable too with the felspar and augite phenocrysts, 

 some of which have been fragmented and corroded. (See Plate V, fig. 1.) 



P. 317 (630). Kulaite. — This rock is of a similar texture and constitution to 

 P. 325 but differs in the base, being much more evenly fine-grained in texture — 

 practically none of its constituents exceeding cryptocrystalline size and most of the 

 material is hyaline. The rock is sempatic and the phenocrysts are corroded and broken 

 as in the previous slide. The pseudomorphs are of exactly the same type, but the 

 proportion of unaltered hornblende is greater. The pleochroic augite is undergoing 

 chloritic decomposition, and magnetite in this slide occurs as a primary phenocrystalline 

 constituent as well as in the ground-mass, where it is developed both as a primary and 

 secondary mineral. 



Felspar is a minor constituent so that this rock so if a somewhat more basic type 

 than the previous. 



The four rocks just described are porphyritic hiatal with phaneric phenocrysts. 

 The texture is hypocrystalline inclining to vitrophyric in fabric. The phenocrysts of 

 hornblende in each show a remarkable variety of forms of alteration, depending, it would 

 seem, on the amount of iron in the original hornblende. The glassy base of each contains 

 a felspathoid, probably leucite. 



These rocks lie in the trachydolerite group and have affinities with the camptonites, 



