. = 
| aw * e Pe. ey 
» Sass . bs 
2 Geological Structure of Keweenaw Point. 75 
Numerous explorers had visited Isle Royale anterior to my sur- 
_ Vey, but mining operations had not been entered upon to any ex- . ° 
_ tent on account of the difficulties arising from some official mis- 
_. understanding as to permits for leases. Dr. Locke had selected 
- Some veins and beds of copper for the Ohio and Isle Royale Com- 
pany, and explorations were going on to determine the probable 
value of several veins. 
There are two kinds of veins on this island, as before men- 
tioned. The widest are those near Rock Harbor. ‘They are 
thick beds of solid epidote rock filled with small spicule of cop- 
’ per, there being from eight to ten per cent. of the metal in the 
gangue. These beds dip but slightly from the horizon, rarely 
more than 15 or 20°, and crop out on the south side of the island 
a few feet above the surface of the Lake. Beneath the copper- 
bearing bed of epidote, which is a foot in thickness, is a large bed 
of barren epidote rock, six feet thick, and very hard. Trap rocks 
overlie the whole, forming bold precipitous shores. No mining 
Operations have yet proved the extent of these cupriferous epi- 
dote rocks, but they are exposed to a sufficient extent to render it 
probable that they will prove of value. 
Another set of true veins occur, cutting the trap rocks nearly 
at right angles, and traversing the country. These veins are gen- 
erally narrow, and are filled with datholite, prehnite, and native 
copper. The datholite is very abundant, and may prove of eco- 
homical importance either as a flux for copper ores, or as a material 
Suitable for the manufacture of borax. One of the locations 
of the Ohio and Isle Royale Company was named, by Mr. J. H. 
Blake, Datholite, on account of the abundance of that mineral in 
the veins of copper. 
At Todd’s Harbor, mines have been opened, and a considera- 
ble quantity of native copper has been obtained by Mr. McCul- 
loch. Other veins have been opened at Scovill’s Point, but as 
yet none of the veins on the island have been sufficiently proved 
to authorize the erection of permanent works for mining and 
Smelting. One vein of each kind, opened to a considerable depth, 
would give much valuable information concerning the perma- 
nency of the veins, and determine whether they widen and en- 
tich or not. 
_ It is extremely difficult for any one to decide absolutely on the 
Value of a metalliferous vein, and it is only possible to form an 
approximate estimate where all the conditions of the problem are 
Capable of being determined, and it is rarely the case that we 
have any thing more than a superficial view of the contents of a 
vein. It has been proved by a writer in the French Annales des 
Mines, that in Germany and France only one-twentieth of the 
Mines surveyed and recommended by the Royal Engineers of 
mines have paid a profit to the stockholders, heuce we should re- 
