172 
AMERICAN GEOLOGY. 
that two of the horizontal galleries penetrate the borders of the 
sandstone, but are soon discontinued. It is proved by many 
observations, that an ore or metal in passing from one rock to 
another is greatly diminished in quantity, or the contrary. In 
the case before us, the copper, when it passes into the sand¬ 
stone, becomes a mere thread or string; and though the fissure 
may exist, or may have been formed, the metal it contains bears 
no comparison in quantity to that in the trap rock. The 
fissure of the sandstone, if it equals in width that of the trap, 
will be filled mostly with veinstone. It can not escape the 
reader’s notice, that this fact—the change which a vein under¬ 
goes in passing from one rock to another—is one of the most 
interesting, as well as important, in all mining operations 
where two rocks bear the same relations as those of the trap 
and sandstone of lake Superior. 
The traps, however, are not equally productive in metal; 
and there appears to be as much difference in the three kinds 
of trap, the soft amygdaloids, hard granular trap, and the 
greenstone, as between the most productive trap and sandstone. 
The soft amygdaloids contain copper lodes, but they are thin, 
branching and scattered; while in the hard greenstone the 
veins are contracted and pinched out. In the fine granular sub¬ 
crystalline trap the veins reach their maximum of excellence. 
The gangue of the copper lodes is in keeping with the rock 
which contains them. The zeolites, prehnite, laumonite, &c., 
are minerals of trap rock; so in their geological position they 
become the veinstone of the copper, and other metals of the 
rock. Calcspar is also a veinstone, but it is also an associate 
of zeolite. It appears from this fact that the veinstone is 
derived from the rock containing the vein. 
The veins of native copper vary in width. I may cite, as 
an illustration of the fact, the well known Cliff mine, situated 
on Keewaunee point, three miles from the lake shore. The 
outcrop of the vein is in a cliff of greenstone. It is only two 
inches wide in prehnite as its gangue, but an exposure on the 
