348 O. A. Derby — Association of Argillaceous 



oughly sound, bluish, imperfectly laminated phyllite of which 

 numerous blocks have been built into a wall at the mine-house. 

 Some blocks have in the joint planes handsome plates of specu- 

 lar iron like those of the G-uia clay. The rock is for the most 

 part mottled with large yellowish blotches suggestive of 

 enclosed crystals or pebbles, and on a polished cross section 

 shows a minute banding with sheared eyes, corresponding to 

 the blotches, suggestive of a sheared porphyritic rock. The 

 most massive specimen shows evidence of shearing in two planes 

 nearly at right angles to each other, and, in this, part of the 

 original structure which, in the most of the specimens, has 

 been obliterated by the two systems of shearing can still be 

 observed. The appearance under the lens is that of an altered 

 diabasic rock sheared to the point of obliterating the rectangu- 

 lar outlines of the feldspars without completely destroying 

 them. The spots above referred to have the aspect of pheno- 

 crysts of feldspar that have lost something of their sharpness of 

 outline. Microscopic sections show a general sericitic mass 

 blackened in irregular bands and patches with a line dust of 

 iron ores which on being dissolved with acid leave whitish 

 aggregates of a titanium mineral (anatase?). The heavy 

 residue consists of a great abundance of iron and titanium 

 minerals, the latter only rarely showing well-defined forms of 

 rutile and anatase. The only other elements recognizable are 

 rare grains of tourmaline and small prismatic fragments, rarely 

 perfect crystals, of monazite. Some of the grains referred to 

 this mineral may possibly be zircon, but if so they are perfectly 

 fresh (not rolled), but indistinctly characterized. The original 

 rock was apparently a basic eruptive very rich in iron and titani- 

 um minerals (ilmenite ?) and with rare phenocrysts of the more 

 acid element. So far as can be made out, this rock would on 

 decomposition give exactly the characters of the red clay of the 

 G-uia. The principal difficulty in the interpretation of this 

 rock and of the red clay of the Gruia as a sheared and meta- 

 morphosed basic eruptive is the presence of monazite, which 

 thus far is only known in situ in the acid eruptives or their 

 metamorphosed representatives. 



Two other rocks from the same region present certain char- 

 acteristics in common with the one above described and if, as 

 seems probable, they can be genetically connected with it, will 

 serve to establish a graduation into a more acid type on the 

 one hand (thus removing in part the difficulty above noted), 

 and on the other hand into a better-defined basic type. From 

 near the Serra do Gigante, several miles to the northward of 

 Sao Joao da Chapada on the road from Diamantina to Grao 

 Mogol, my friend and companion, Dr. Francisco de Paula 

 Oliveira, brought a loose block of a peculiar cyanite-phyllite 



