76 a C. T. Jackson on the Geological Stritvure, ge. 
pee persons about to engage in an adventures that shir 
chances of success are only about five per c 
When a mine is well proved it gaeenlig holds good veins, 
rarely running out in i unless the rock changes, and then the 
vein also*generally alte ms 
How far the native ane of the Lake Superior mines con= 
tinues in depth is yet unknown, but the veins if they traverse sand-. 
stone strata will certainly change i in that rock, and experience has th 
thus far shown that the copper diminishes in that portion of the *--. 
vein which traverses the sandstone. ‘This has been fully proved _ 
at the Copper Falls mines, where a bed of sandstone, seventy-two ~~" ~ 
feet in thickness, has changed the character of the’ lode where 
the vein passed through it, calc spar filling the chasm and. the ©. — 
copper big id disappearing in the veinstone. 
It was hoped that the vein would enrich after it had passed 
Brough the sandstone into the nether bed of trap rock, but it was 
found to be diffused into string veins of little practical impor- 
tance. Owing to the limited extent of the amygdaloidal trap, 
the true or transverse veins are not of great length, two thousand 
feet being perhaps an approximation to their linear extent, though 
it is possible that some may be longer. The idea of tracing a 
vein by its course over an extensive tract of country has proved 
fallacious on Lake Superior, and only the geological character of 
the country can be relied upon asa tolerably correct guide. The 
river beds, Gapescminiiag in the soil, corresponding to the usual di- 
rection of the veins, afford t e best facilities for finding veins, 
and by means of the solar si and magnetic needle, lines of 
contact of the sandstone and trap may be readily found, and = 
amygdaloid is formed at those junctions. It was o observed in 
first visit to the Lake, that the productive copper veins occur 
where there are the greatest number of alternations of sandstone 
and trap rocks, as shown by the diagram exhibited to the Section. 
No less than six alternations of these rocks were observed near 
Copper Falls and Lake Superior mine in my surveys during the 
summers of 1844 and 1845. Subsequent researches have con- 
firmed this observation. 
From these data we should expect copper veins at the line of 
contact of the sandstone and trap on Isle Royale, but thus far in 
only a few places have the rocks been uncovered in the vicinity 
of the junction—some loose masses of native copper found on 
the shore of Siskawit Lake and the veins at raerneg Cove being 
the only facts obtained in confirmation of this o 
In the hard columnar and compact trap Horsf is 5 little hope of 
finding valuable Mena for only narrow and tightly pinched seams 
of copper have t hus far been found in these rocks. 
On the Ontonagon river there are several veins of copper that 
run with the “ ounce - They are now in course of trial, and 
