344 



0. A. Derby — Association of Argillaceous 



quartz. Throughout the entire length of the cutting, a dis- 

 tance of over 500 meters, runs a sharply-defined layer from 1 

 to 2 meters thick of argillaceous material with quartz which 

 has received from the miners the name of Guia (Guide), 

 because, as they state, diamonds were to be looked for. below 

 (or in front) and not above (or behind) the outcrop of this 

 layer. 



This layer consists principally of a mass of dark red foliated 

 clay which is evidently a decomposed schist stained with oxide 

 of iron. In several places that afford access to its interior, the 

 central portion is seen to be occupied by an irregular but 

 apparently continuous mass of vein quartz, that sometimes 

 swells out so as to occupy half or more of the thickness of the 

 layer, being in other places reduced to a thickness of a few 

 centimeters only. Associated with the quartz is a pure white 

 granular kaolin which in places is intimately mixed with small 

 angular granules of quartz in a way that suggests a decom- 

 posed granite, but in general forms pockets or a selvage in or 

 around the more massive portions of the vein. Outside of the 

 kaolin, when this forms the marginal portion of the vein, or 

 inclosed as angular fragments in its mass, is a minutely banded 

 reddish, or yellowish, micaceous clay that appears to differ 

 from the red clay composing the greater part of the mass of 

 the Guia only in the absence of the intense staining with 

 oxide of iron. The relations of the white kaolin with the 

 banded clay is well shown on the scraped surface of a block 

 that was brought away, and represented about one-fourth the 

 natural size, in the accompanying figure, in which the dotted 



portion represents the granular white kaolin and the shaded 

 portion the banded clay. The unshaded streaks in the latter 

 are also of white kaolin that differs from that of the vein in 

 the less distinct granulation, but which in places, as at the lower 

 right hand side, appears to be flame-like apophyses from it. 



