Reviews — Plagioclase-nepheline Granitoid liocJcs. 185 



account is given of work carried out by the Wallaroo and Moonta 

 Mining and Smelting Company, Ltd., for the welfare of their 

 employees, which is of a most elaborate and well-organized nature, 

 and conditions there appear to be ideal. 



The Classification op the Plagioclase-nepheline Granitoid 



Rocks. By A. Lacroix. Comptes Rendus, t. 170, 1920, p. 20. 

 rpHE terms essexite and theralite have suffered somewhat from lack 

 -*- of definition and discrimination in their use. Rosenbusch 

 suggested the term theralite for plagioclase-nepheline rocks of 

 plutonic habit, and chose as the original type a rock from the Crazy 

 Mountains, Montana, in which plagioclase was stated to be present. 

 This was a mistake, as the rock proved to be a shonkinite, and 

 recognizing this Rosenbusch afterwards took the unequivocal 

 theralite of Duppau, Bohemia, as the type example. Lacroix, 

 however, proposed in 1902 to use theralite for melanocratic alkali- 

 syenites. He admits that with other petrologists he has applied 

 the term inconsistently, and in an instructive paper he now attempts 

 to put this group of rocks in order once and for all. 



The term theralite he reserves for mesocratic rocks which differ 

 from essexites in having a smaller proportion of potash-felspars 

 (often reduced to a mantle bordering the plagioclase crystals), and 

 in general a higher ratio of nepheline to felspars. In essexites, as 

 in the type-rock of Salem Neck, there is a notable quantity of 

 orthoclase or soda-orthoclase ; interstitial nepheline may be present, 

 and hornblende is a constant ferromagnesian constituent. 



From the theralites proper Lacroix separates two types in which 

 the nepheline-felspar ratio is lower and the plagioclase more 

 calcic than in the normal members of the group. Luscladite, one of 

 these types, is characterized by the general absence of hornblende 

 and the presence of olivine and often of biotite. The Crawford] ohn 

 essexite is a British example, and Tyrrell's h/lite is regarded as a 

 melanocratic facies. The other type, berondrite, is characterized 

 by the presence of elongated crystals of brown hornblende associated 

 with titaniferous augite. The montrealite of Mont-Royal is a coarse- 

 grained melanocratic variety. Theralitic rocks less rich in* alkalies 

 than those already mentioned are the mareugites, which are further 

 characterized by hauyne, and which vary from highly felspathic 

 to almost holomelanocratic types. 



Two rocks are considered as heteromorphic forms of berondrite, 

 since they differ essentially from the latter in mineral composition 

 but not in chemical composition. In mafrdite nepheline fails to 

 appear, its constituents being present in an aluminous soda- 

 amphibole. In Jasinife titaniferous augite contains the missing 

 mineral, which in this case is plagioclase. Fasinite, though com- 

 posed essentially of titaniferous augite and nepheline, thus differs 

 from ijolite in containing abundant normative plagioclase. 

 Bekinkinite is not dissimilar in chemical composition from fasinite 



