ORIGIN OF THE IRON ORES. 83 
be at a maximum near the contact of ores and the adjacent rocks at 
the surface. Iron sulphide is changed to limonite and melanterite, 
some of which is removed in solution. Iron carbonate alters to 
limonite. 
Apatite has been altered to osteolite and leached in the upper 
parts of the ores to a very considerable extent. Frequently the 
entire crystal of apatite has disappeared, its former presence being 
indicated only by the shape of the cavity. Presumably this apatite 
is redeposited below. This does not necessarily mean that the ores 
beneath the surface will run higher in phosphorus than at the surface, 
for the reason that there may be concentrated at the present surface 
a large part of the phosphorus which has come from the erosion of 
the overlying materials — a relatively larger amount of phosphorus 
than has been leached from the present surface and carried farther 
down. 
Erosion has cut down the ore deposits many feet, as shown by 
abundant ore debris on the slopes. The ore is fissured. Extreme 
temperature changes spall off considerable blocks bounded at the 
sides by fissure planes. The ultimate product is magnetic sand, 
abundantly found on the lower slopes. For most of the district this 
erosion has gone on much less rapidly in the ore than in the adjacent 
andesite and limestone, with the result that the ore stands up in con- 
spicuous black masses above the surrounding rocks. In a number of 
places, however, as on the Vermilion, Lindsay, Enterprise, part of the 
Mount Lion group, Comstock, Sunbeam, Wellington, Queen of the 
West, Black Hawk, Pinto mine, Pinto Nos. 3, 5, and 6, Burke No. 5, 
Red Clouds, and Duncan No. 1 claims, the ore has been cut down 
flush with the surrounding rocks. In these places the ore is uni- 
formly softer than where standing up in conspicuous crags. It may 
be that the hard crystalline surface ores, serving more or less as pro- 
tecting caps, have been locally undermined and cut off, leaving the 
underlying softer ore unprotected, with the result that it is cut down 
fully as rapidly as the adjacent rocks. 
There is no evidence that weathering contributed to the deposits 
any considerable amount of ore from adjacent rocks. During erosion 
the magnetic iron of the andesite is concentrated into magnetic sands, 
as is the magnetite derived from the disintegration of the ore deposits. 
If alumina be assumed to remain constant during weathering it will be 
apparent from the analyses of the andesite that a small percentage of 
iron has been lost. With this percentage of loss it would require the 
weathering of a mass 100 feet thick and 0.2 square mile in area to 
yield a million tons of 56 per cent ore. 
Iron is only slightly leached from the limestone during weathering, 
all but a minute part remaining in the residual clay. The residual 
soil from the Homestake limestone has been removed and the ore 
