346 0. A. Derby— Association of Argillaceous 



tions from the latter, and in support of this view is the fact that 

 the white bands are distinctly kaolinitic (though not granulated 

 like the kaolin of the vein), while the colored ones are as dis- 

 tinctly micaceous. Aside, however, from these distinct white 

 bands, there is an obscure lamination visible even in the most 

 highly-stained portions. On a carefully scraped surface cut 

 transverse to the lamination a few scattered rounded eyes of 

 quartz are in some portions visible, which with the banded 

 structure are very suggestive of a stretched quartz porphyry. 

 These eyes occur both in the dark red clays and in the lighted 

 colored ones, but are much more abundant in the latter than 

 in the former. This fact taken in connection with the in- 

 creased staining of the clay towards the margins of the mass 

 and the apparent lack of a physical break between the lighter 

 and the darker colored clays may, perhaps, be taken as an 

 indication that the original rock was more acid in the central 

 and more basic (or at least richer in iron) in the marginal por- 

 tions of the mass. 



On washing, both the dark and the light colored clays give, 

 after a considerable loss as slime, an abundant residue of white 

 micaceous aggregates (in the case of the red clay treatment 

 with acid is necessary) with the aspect of sericite with rare 

 granules of quartz that appear to come exclusively from the 

 eyes. On separation of the mica and quartz by washing or the 

 use of heavy liquids, we find an extremely abundant residue of 

 mixed r utile and anatase (sometimes the one, sometimes the 

 other predominating), which appear to come from original 

 ilmenite represented by abundant black grains in the red and 

 by rather rare, dirty aggregates of ill-formed anatase or rutile 

 in the lighter colored clay. "With the titanium minerals occur 

 rarely minute prisms of tourmaline and prismatic fragments, 

 rarely perfect crystals, of monazite of the peculiar type to be 

 described farther on. All the elements of the residue are dis- 

 tinctly authigenetic and in this respect are in marked contrast 

 with those of the adjoining quartzite, which with the same 

 titanium minerals (with the exception of ilmenite) and tourma- 

 line in perfectly fresh crystals contains an abundance of well- 

 rolled zircons. 



The Guia is thus a composite body that consisted originally 

 of a vein, or dike, of a quartz-feldspar rock, presumably 

 pegmatitic granite, injected into a micaceous schist that may 

 have been originally either a sedimentary shale or a sheared 

 eruptive. The appearance of the mass is exceedingly dike-like, 

 but if a dike, it must have been injected along a bedding 

 plane, or so nearly coincident with one as to simulate an inter- 

 callated layer. The characters above given of apparent por- 

 phyritic structure, absence of distinctly clastic elements, and 



