and their Contact Zones. 75 



mine — partly composed of carbonates of lime, iron, and 

 magnesia, derived, no doubt, from the decomposition of the 

 soda-lime felspars and hornblende of the diorites. 



In order to ascertain what inferences may be drawn from 

 the reefs themselves, I propose to consider a few of the mines 

 as to which I have made notes. 



Eureka Reef. 



This quartz vein is situated on a spur falling towards 

 Eureka Creek. It has been somewhat extensively worked 

 by adits put in from a gully on the south side of the spur. 

 The last mining operations were carried on at the west side, 

 where an adit was partly driven in intensely hard quartz 

 diorite, but was abandoned before reaching the reef. No. 1 

 adit, which I examined, follows the course of the reef, which 

 winds much, varying in direction from N. to N. 25° W. 

 Its average direction is probably N. 20° W. The underlay* 

 in the adit is westward at about 80°. I found the reef to 

 vary very considerably, both in underlay and in strike. In 

 following the reef the adit was at first on the east side, but 

 near the end crossed and followed the west side — that is, the 

 " hanging wall." At about this place was, I believe, the " shoot 

 of gold" in which the main shaft was sunk. To the north of 

 this point the reef is very narrow, or even only indicated 

 by the walls and an inch or two of " mullock." In places 

 there are small bulges of quartz. The same features occur 

 in passing up the higher stopes to the surface. The reef is 

 entirely in a quartzose diorite, but there are traces of con- 

 tact schists close at hand at the Black Prince Mine, and the 

 main contact is at a distance of 20 chains west. 



The footwall of the Eureka reef is a dull greenish or 

 almost colourless compact rock of a somewhat felsitic 

 character. A microscopic examination showed me that it 

 is composed of aggregates of quartz granules similar to 

 those seen in the quartz diorites, filling in spaces between 

 altered minerals, having the shape of felspars. These latter 

 are composed of minute scales and twisted flakes, and more 

 rarely of plates of a micaceous* mineral — perhaps muscovite 

 if the crystals were originally orthoclase, or paragonite or 

 margarite if they were soda or soda-lime felspars. 



* I here use the mining term " underlay" as indicating the angle formed 

 by the vein with the horizon, and the term " dip" solely in reference to the 

 angle of descent of the shoot of gold in the direction of the strike of the reef. 



g2 



