0. A. Derby — Magnetite Ore Districts in Brazil. 319 



where a decomposed intrusive sheet of a highly micaceous 

 rock is intercalated in the Carboniferous sandstone and shale. 

 This includes lenticular nodules that have escaped decomposi- 

 tion varying in size from that of an egg to that of a bushel 

 measure. These consist of the orthoclase-pyroxene (with 

 subordinate apatite) and pyroxene-apatite (with subordinate 

 orthoclase) combinations identical in aspect, both macro- and 

 microscopically, with the rocks from the ore locality at the 

 same place and from the Modesto locality at Jacupiranga, At 

 this place, owing to the decomposition of the enclosing rock, 

 no idea can be found of the origin of the nodules which 

 appear like transported blocks. At another point where the 

 rock is better preserved small aggregates of the same nature 

 seen in microscopic slides are evident segregations. The 

 enclosing rock has a porphyritic structure with large pheno- 

 crysts of nepheline, apatite, green pyroxene (acmite) and 

 biotite (extremely abundant) in a fine grained ground- mass too 

 much altered for determination. It had thus the structure and 

 aspect of a phonolite although no feldspar could be detected 

 in it* and the biotite is much more abundant than is usual in 

 the ordinary phonolites. In composition it recalls a coarse 

 holoerystallirie nephelinite found in loose blocks at Jacupi- 

 ranga. A comparison is also suggested with some of the rocks 

 from the Azores and Masai Land called acmite-trachyte by 

 JVIugge. A rock of this last type with phenocrysts of sani- 

 dine brown pyroxene and hauyne in a felt like ground- mass of 

 orthoclase, acmite and possibly nepheline (since the former 

 gelatinizes freely) occurs in considerable masses at Jacupiranga 

 somewhat to the westward of the ore district. 



The above observations indicate in the phases of laurvikite 

 here described a genetic relation with nepheline-bearing rocks 

 and a tendency to a schistose structure, and to abrupt changes 

 in mineral composition, especially in the direction of phos- 

 phatic and basic segregations, that is found to a greater or less 

 extent in other phases of the same type at various localities in 

 Brazil. Thus, at two points in the Jacupiranga district and at 

 Cabo Frio, a coarse granitoid laurvikite with rare grains of 

 nepheline and abundant biotite is associated with foyaite 

 masses. A fine grained phase of the same type found in con- 

 siderable bodies in various outlying hills about the Jacupiranga 

 ore district and in a few loose blocks within it, show a tend- 

 ency to a linear arrangement of the bisilicate elements (pyrox- 

 ene, hornblende and biotite) that gives a gneissoid aspect to 

 the rock. The same tendency is seen in an angular fragment 



*This maybe due to decomposition though as the nepheline is still recognizable 

 it seems singular that orthoclase should have been less persistent. In the segre- 

 gations uepheline is totally excluded and primary biotite is rare. 



