Rocks with Quartz Veins in Brazil. 349 



fall of enormous crystals of cyanitein an imperfectly laminated 

 groundmass of sericitic appearance. The rock is free from iron 

 ores, but extremely rich in minute aggregates of rutile and of 

 extremely minute prismatic needles that are referred to mona- 

 zite. The original rock was evidently a highly aluminous 

 (feldspathic) one, with an abundance of a titanium mineral 

 (ilmenite ?) and of monazite, in which through shearing and the 

 development of secondary minerals, all traces of the original 

 structure has been obliterated. A very similar rock, but lack- 

 ing the monazite, was found by Dr. Hussak on the 11 ha do Fogo 

 in the river Sao Francisco in front of the town of Joazeiro, 

 where it presents the aspect of a dike in gneiss. 



Some 15 or 20 miles to the southward of Sao Joao da 

 Chapada, near the fall of the river Dattas close by the town of 

 Gurvea, another imperfectly laminated phyllite was found out- 

 cropping in the road that strongly resembles that of Sao Joao 

 but differs principally in the absence of monazite, in the pre- 

 servation of the original rectangular outlines of the white 

 mineral transformed into sericite, and in the great quantity of 

 tourmaline that is evidently an element introduced in the 

 metamorphism of the rock. In addition to the fine iron dust 

 that on removal by acid leaves a dust of formless aggregates of 

 titanium, the rock contains magnetite in considerable-sized 

 grains and octahedral aggregates of an altered titanium mineral 

 that strongly resembles those of decomposed perofskite 

 described by Hussak from the diamond gravels of Agua Suja 

 (Neues Jahrb., 1894, ii, p. 297). Judging from the sharply 

 cut rectangular outlines of the areas of sericite free from the 

 iron dust, the original rock must have been largely composed 

 of lath-shaped crystals like those of the plagioclase or melilite 

 of diabasic or basaltic rocks, and with this conclusion the great 

 abundance of the iron and titanium dust, that may be presumed 

 to come from original ilmenite, and of magnetite with an octa- 

 hedral titanium mineral, is in accord. The shearing has not 

 been sufficient to obliterate the form of the white element and 

 appears to have been anterior to the introduction of the tour- 

 maline that lines the joint planes and has invaded many of 

 the rectangular sericitic areas without passing beyond their 

 limits. 



There can be no reasonable doubt but that the rock last 

 described is a metamorphosed and moderately-sheared basic 

 eruptive and when it is recalled that " mixed dikes " have 

 been described (see Zirkel, Petrographie, i, p. 784) in which a 

 granite-porphyry passes to a type that has been described as 

 diabase or melaphyre, the hypothesis that the two rocks above 

 described and the decomposed material of the Guia may be 

 genetically related is not so extravagant as at first sight appears. 





