212 Derby — Notes on Certain Schists of the Gold and 



like No. II, it is very poor in lime and magnesia and that it is 

 composed essentially of a sericitic mica heavily charged with 

 hematite dnst. The two rocks are therefore closelj 7 similar, 

 chemically as well as mineralogically, and it can safely be 

 affirmed that the principal differences revealed by a complete 

 analysis would be only such as correspond to the presence of 

 the characteristic elements of tourmaline and to a greater 

 abundance of titanium and probably also of iron oxide, both in 

 the free state. These two rocks also agree in presenting on a 

 polished section transverse to the plane of shearing, appear- 

 ances of original structure that in the Dattas rock are perfectly 

 well-defined but somewhat indistinct in the other. This 

 appearance is that of the mottled aspect of a rock composed of 

 an intimate mixture of white and colored elements, as, for 

 example, a diabase or a basalt. In the Dattas rock the minute 

 white areas are perfectly defined rectangles like those of the 

 feldspar of the rocks mentioned, while in that from Sao Joao 

 da Chapada (No. II) they appear to have lost their original 

 sharpness of outline through shearing. Under the microscope, 

 these rectangular areas of the Dattas rock are seen to be com- 

 posed of a sericitic aggregate free from the iron and titanium 

 dust with which the rest of the section is thickly sprinkled. 

 On dissolving out the iron, the distinction between the white 

 and colored areas disappears and the whole slide presents a 

 sericitic aggregate sprinkled with a dirty white dust of titanium 

 oxide, rutile or anatase, which before had been concealed by 

 the iron. The appearance is that of a basaltic rock composed of 

 a well crystallized white element (plagioclase or melilite) in a 

 groundmass containing bisilicates (pyroxene?) or in a basic 

 glass rich in iron and titanium oxides, which on alteration has 

 settled in the place of the original minerals, or glass, but with- 

 out invading the areas of the white element. In addition to 

 this secondary iron (hematite) and titanium dust, the rock also 

 contains these two elements as primary constituents in the 

 form of crystals, often of considerable size, of magnetite and 

 of minute octahedrons of an altered titanium mineral that was 

 most probably perofskite. 



The peculiar structure and characteristic primary accessories 

 of this rock are strongly suggestive of an original basaltic 

 character and, in view of the perofskite, most probably a 

 melilite-basalt. On this hypothesis, however, the original rock 

 must have been rich in lime and magnesia and its present 

 chemical and mineralogical composition can best be explained 

 by the hypothesis that before metamorphism it had been 

 decomposed and leached in situ, the resulting residual clays 

 and oxides retaining their original positions without mingling. 

 A confirmation of this hypothesis is apparently to be found in 



