MR. 0. A. DEUBY ON NEPnELINE-IlOCKS IN BRAZIL. 2G3 



phonolito ; it is usually (juitc distinct in the large simple masses 

 of the foyailc, and in the central crystal of the compound groups, 

 T\-hile it is wanting or less marked in the peripheral members of 

 the same. Those groups, as a whole, however, show zonal structure 

 in the disposition of the peripheral i^seudo-crystals in one or more 

 rings around a central one, the interspaces being occupied by 

 groundmass extraordinarily rich in basic elements (bisilicates, me- 

 lanite, and magnetite). In one specimen of phonolite tiuidal 

 structure in the groundma.ss surrounding the pseudo-crystais was 

 observed. Occasioually a block is found so crowded with these 

 compound pseudo-crystals, standing out in relief through weathering, 

 that the surface resembles that of a gigantic pineapple. Usually, 

 however, they are rather distantly scattered, one or two only ap- 

 pearing in the sjjace of a s<|uare metre. The simple crystals attain 

 a diameter of 10-15 centimetres, which is more than doubled in the 

 compound groups. 



To account for the formation of these singular masses Dr. Hussak 

 rejects, with good reason, the hj^j^wthesis that they are inclusions of 

 pre-existent foyaite ; and, arguing from their complicated structure 

 against the hypothesis of pseudomorphism by alteration, he looks 

 upon them as true pseudo-crystals, representing a tendency to the 

 formation of leucite under physical conditions unfavourable to the 

 complete development of that mineral. jS^one of the sections, thus 

 far examined, have shown unmistakable leucite either in the pseudo- 

 crystals or the groundmass, although rarely decomposition-products 

 are observed that may possibly represent it. That these aggregates 

 are intrateliuric is shown by the phonolite dyke at Tingua and the 

 foyaite sheet at Po^os de Caldas ; while the fact that the leucite 

 form, skeleton structure, &c. are given by a second generation of 

 orthoclase aud nepheline around an aggregate of phenocrysts, indi- 

 cates their completion in the later stages of the consolidation of the 

 magma, and eonsequently a comparatively superficial origin *. 



Diecke has suggested that under increased pressure the formation 

 of leucite is impossible t. It may be supposed that these aggre- 

 gates have been formed in a magma that had come to rest under a 

 pressure sufficiently reduced to allow of the tendency to the formation 

 of leucite, but still too high to permit of its comjjlete development, 

 so that, after sketching out the tbrm and iuterual structure of leucite, 

 the material crystallized as orthoclase and nepheline. It is diffi- 

 cult, however, to conceive how a mere tendency, without the sub- 

 stance, could produce such perfect crystalline forms, and it may be 

 suggested that perhaps crystals of leucite, exceedingly rich in inclu- 

 sions (phenocrysts from the surrounding magma), were actually 

 formed, but that, before the complete consolidation of the magma, 

 some change of conditions brought about, through magnetic action, a 

 pseudomorphosis of the leucite molecule into orthoclase and nepheline, 



* The same conclusion is indicated by the sanidinite ' bombs ' brought from 

 Vesuvius by the late G. vom Rath, in -wLich, according to Dr. Hussak, identical 

 pseudo-crystals occur, I have been unable to find any reference to these bodies 

 in the writings of Vom Rath at hand. 



t Neues Jabrb. BeUage, vol. vi. p. 226. 



