1S8 0. A. Derby — Gold-bearing Lode of Passagem. 



The almost absolutely fresh feldspar crystal (3) in fig. 2 

 lying in the quartz hand B is, on the face photographed, com- 

 pletely surrounded with quartz free from tourmaline and 

 pyrite, in a zone of the width of two centimeters or more, 

 while at the back of the block scattered crystals of these min- 

 erals can be traced up to its margin, and here a portion of the 

 crystal shows incipient sericitization. Its neighbor 2 is still 

 comparatively fresh, while 1, which abuts directly on a strongly 

 mineralized part of zone C, is so completely altered that all 

 traces of the original cleavages have been obliterated. In 

 many cases it is clear that the mineralizing agent has only 



Fig. 3. 



reached one face of the feldspar crystal and that this contact 

 has been sufficient to effect the alteration of the latter. This 

 is well shown in fig. 1 in the case of the two grouped crystals 

 at the middle of the upper margin in which narrow bands 

 of pyrites (distinctly seen in photograph) are interposed 

 between the two feldspar individuals and between the left- 

 hand one and the quartz, while at the right the contact 

 between the quartz and the inclusion is perfect. All the other 

 inclusions on this slab show the same phenomena of a narrow, 

 almost linear, mineralized strip (with pyrite predominant over 

 tourmaline) along one face and a sharp cut contact with the 

 quartz on the others. Evidently the mineralizing agent 

 entered through small fissures which, on meeting a feldspar 

 crystal, passed along one of its faces, which, in the case of all 

 those shown in fig. 1, was always on the same (left) side. 



