

Rooks with Quartz Veins in Brazil. 347 



the presence of monazite, all favor the view of an eruptive 

 origin and appear to be of more weight than those of schistose 

 structure and apparent interbedding with the quartzite which 

 are the most important that can be cited in favor of the oppo- 

 site view. This question will be discussed farther on. 



For an attempt to reconstruct the original rock of the colored 

 portion of the Guia we have the positive indications of a schist 

 free or nearly free from quartz and rich in sericite (or a mica- 

 ceous mineral resembling it which for convenience will be 

 called by that name), with an abundance of iron and titanium 

 minerals, tourmaline and monazite, as characteristic primary 

 accessories and with an entire lack of distinctly clastic elements 

 which in a series with the degree of metamorphism of that 

 about Diamantina could only be rolled zircons or regenerated 

 tourmalines.* For the purpose of attempting such a recon- 

 struction, specimens were collected of such rocks as from their 

 general aspect might be suspected of representing the original 

 type from which the Guia clays might be derived. From the 

 numerous specimens examined from various localities, the com- 

 parative freedom from quartz and the abundance of sericite, 

 of iron and titanium minerals and of tourmaline prove to 

 be common characteristics of the phyllites of the region, which 

 can only be distinguished as presumably original sedimentaries 

 or eruptives by the presence or absence of distinctly rolled 

 zircons (often of a size and abundance that seems extraordinary 

 in rocks presumed to represent fine-grained sediments) and of 

 monazites or zircons, or both, with the aspect of original 

 primary accessories. 



The rock which from its position and aspect seemed to be 

 the most promising was a partially decomposed greenish schist 

 found in loose blocks at the foot of the western wall of the 

 cutting, and which, judging from appearances, was thought to 

 have fallen from a layer somewhat similar to the Guia and only 

 a few dozens of meters below it in the same geological series. 

 This, however, proves to be characterized by an abundance of 

 well-rolled zircons and of regenerated tourmalines and by an 

 entire absence of monazite. 



Another rock from the immediate vicinity and presumably 

 from the deep rock cutting (mainly in hard itacolumite) for 

 the drainage canal of the Duro mine (seen in the background 

 of the above cited figure in the Journal of Geology), is a thor- 



* See Derby, On the accessory elements of itacolumite, etc., this Journal, 

 March, 1898. In this paper it was suggested that the lazulite that occurs quite 

 frequently in the quartzites about Diamantina might have been derived from 

 original clastic phosphates. This view is confirmed by an examination of more 

 abundant material in which distinct but partially altered and corroded monazites 

 and xenotimes were found in the heavy residue of nodules rich in secondary 

 lazulite. 



