232 W. LINDGREN CALIFORNIA GOLD-QUARTZ VEINS. 



Under exceptional circumstances rock containing as little as one or two 

 dollars of gold to the ton will pay. In the deep mines the tenor of the 

 extracted ore is usually from five to twenty dollars. 



In wider veins a small streak near one of the walls will sometimes con- 

 tain the pay, while the rest is comparativeh^ barren. Equal distribution 

 of value in cross-section is, how^ever, common enough. Considered in 

 projection on the plane of the vein, there is rarely an equal distribution 

 of the gold over large surfaces. The richer ore is concentrated in bodies 

 and masses, which sometimes may be wholly irregular, but which usually 

 show more or less regular outlines. These richer masses are called chutes 

 or chimneys, and appear on the plane of the vein in long-drawn linear 

 or elliptic form, with a dip which usually is above 45 degrees. Flat ore- 

 chutes occur, however, as, for instance, in the Idaho mine. Grass Valley. 

 Their width ranges from a few feet up to several hundred, and their 

 length may exceed 2,000 feet. It is not uncommon to find one of these 

 ore-chutes give out in depth, but another Chute will then probably be 

 found in some place below it, if a thorough exploration is carried out. 

 It is a practical rule in many districts, and one which holds good in a 

 remarkable number of cases, that the chutes dip to the left w^hen one is 

 standing on the apex and looking down along the dip. The explanation 

 of the ore-chutes is difhcult. They may, as F. Posepny and others have 

 suggested, simply indicate the direction of least resistance for the gold- 

 bearing solutions. This explanation is not entirely satisfactory, for in 

 the intervals between the chutes it is by no means the rule to find the 

 walls shut down tight. On the contrary, it is common to find the barren 

 vein between them as wide if not wider than the rich vein in them. An 

 increase in the quantity of the sulphurets always accompanies the in- 

 crease of gold in the ore-chutes. 



No gradual decrease in the tenor of the ore takes place with increasing 

 depth : on the whole, the character remains constant. Individual ore- 

 chutes may be exhausted, but others, as a rule, are found below them. 



Certain veins show no large bodies of milling ore at all, but coarse 

 gold concentrated at certain points ; such deposits are called " pocket 

 veins." Small seams may sometimes carry a surprisingly large amount 

 of gold. Intersection of seams or veins often, but by no means always, 

 produce pockets or ore-chutes. 



The Alteration of the Country-rock. 



The study of the changes and alterations which the rocks adjoining the 

 fissures have undergone is a subject of the highest importance, for in this 

 way a closer insight into the genetic processes of the vein may often be 

 obtained. 



