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



white element appears to have been feldspar. This resembles 

 the decomposed schist fragments contained in the ore breccia 

 and like them is characterized by an abundance of zircons 

 of peculiar type. In the midst of these schists appear 

 irregular bands and patches (dykes?) of purely micaceous 

 material exactly resembling the iron-free portions of the erup- 

 tive rock of the breccia and like it enclosing fragments of 

 schist. The mica-apatite schists also contain several streaks of 

 a much whiter more compact material that look exceedingly 

 like dykes but may perhaps be particularizations. In these 

 mica and apatite are the only recognizable constituents, but it is 

 tolerably certain that a large part of their mass was originally 

 feldspar. One of these streaks about 5 cm wide has a central 

 line of rounded aggregates, the size of a walnut, of magnetite 

 and apatite exactly resembling, on a small scale, the bowlders of 

 the superficial ore deposit and the aggregates of the large dyke 

 (p. 12). Large blocks of magnetite scattered about the neigh- 

 borhood of the cutting indicate that larger aggregates occur 

 though none were seen in situ. This little dyke or streak 

 shows that considerable aggregates of magnetite do actually 

 occur in a rock which is presumably a phase of laurvikite and 

 thus permit the reference to that type of the large decomposed 

 dyke with its workable ore bodies. 



The decomposed schist of the ore breccia gives on washing 

 an abundant slime of mica fiakes and kaolin with a residue of 

 microscopic zircons* of peculiar type and rare grains of unde- 

 composed orthoclase and aconite thus indicating an original 

 orthoclase-pyroxene rock. Just such a rock, characterized by 

 the same peculiar type of zircons, is found in a sound state in 

 the schist hill (Morro de Area Preta)f at Jacupiranga men- 

 tioned on p. 800. It here cuts across and is insinuated between 

 the layers of an ordinary metamorphosed Cambrian (?) schist 

 containing quartz, mica, tourmaline and staurolite, in such a 

 manner as to form an extremely curious complex of regular 

 alternating layers, often only a few millimeters thick, of erup- 

 tive and rudimentary material. This schistose phase of laurvik- 

 ite occurs again in a low ridge at a place called Modesto a 

 kilometer or so above the Morro de Area Preta, but here it is 

 itself included in a breccia of which the matrix is the pyroxene- 

 apatite (with subordinate orthoclase) phase of the same rock 

 similar to that already described from Ipanema. 



Indications of the genetic relations and geological age of this 

 last phase are afforded by a cutting on the tramway at Ipanema 



* These are complicated crystals of the Miask type in strong contrast with the 

 simple prismatic forms found in many scores of washings from the ordinary feld- 

 spathic rocks such as gneiss, mica schist, granite, syenite, foyaite, phonolite, 

 diorite, etc. 



f Black Sand Hill from the abundance of magnetite in the stream at its base. 



