Notes on J]Ioi}chiquite Dykes. 11 



visit, and the quartz across its full width showed galena, blende, and 

 pyrite with just occasional colours of gold. Scattered through the 

 milk-white quartz were dark fragments of the country rock, angular 

 in shape, of all sizes, and with definite, unabsorbed boundaries found 

 in fissure filled reefs. Between this reef and the dykes there is 

 thoroughly brecciated material, consisting of pieces of slate, reef 

 quartz and lava. Some of the quartz has lost the milky appearance 

 and become vitreous, a.nd there are greenish patches near the lava 

 pieces coloui-ed l)y the serpentinous material. This material is typical 

 breccia, mineralised, and very rich in gold. 



Pieces broken from right in the side of both lavas showed gold. Mr. 

 Rogers, the manager, gave me a piece of the eastern of the two 

 branches, showing a coarse shot of gold quite obviously detached from 

 any quartz whatever. Examination with a pocket lens showed that the 

 rock immediately around the gold was different from the main mass 

 of monchiquite. 



A thin slice of this rock, cut through visible gold a little distance 

 from the one coarse shot, was prepared, and revealed the fact that the 

 rock actually containing the gold itself was a piece of breccia set in tlie 

 dyke material. The bulk of the gold is mixed with dark, opaque 

 material, which looks decidedly carbonaceous. This dark patch also 

 contains small pieces of a white micaceous mineral, calcite and slaty 

 material. One fragment of gold is detached from the opaque material 

 and set in a lump of fine sandstone, but has several veins of secondary 

 calcite leading up to it. The whole is surrounded by fragments of 

 micaceous slate and sandstone. These fragments have apparently been 

 caught up by the lava in its intrusion, and cemented well into the lava 

 during consolidation. The detached fragments of lava in the breccia 

 indicate earth movements subsequent to the consolidation of the lava. 

 These may have followed the consolidation immediately, resulting from 

 the same rock stresses involved in the intrusion itself. 



A thin slice (Plate I., Fig. 2) of another specimen of the contact 

 material was prepared, cut transversely to the contact, and through 

 visible gold. This section clearly shows crustification parallel to the 

 direction of the lava. The gold in the hand specimen could be dis- 

 tinctly seen lying between crustified bands for a length of about four 

 millimetres before the slice was cut. Under the microscope, pieces of 

 slate, galena, and opaque material are seen between the cruslitied 

 bands. Mixed with the opacpie matter and also with the galena is 

 gold. As before, the opaque matter lookss decidedly carbonaceous. This 

 crustification clearly shows that mineral-bearing siliceous and cal- 

 careous solutions have traversed the side of the lava, subsequent to 

 the lava. The presence of carbonaceims material suggests that it 

 was the precipitant of the gold. 



