ORIGIN OF THE IRON ORES. 89 
has shifted down the slopes makes it impossible to determine the true 
outline of the deposit Avithout sinking pits. Where iron-ore frag- 
ments are found in the wash at the surface it may be assumed that 
they came from farther up the slope. Following the wash up the 
slope, a point will be reached where no iron fragments are to be seen, 
usually indicating that the upper edge of the deposit has been passed ; 
but it has been found in a few localities that the overwash from the 
upper slopes has been so thick and heavy that it completely obscures 
the upper side of the ore deposits, and the first iron-ore fragments 
appear some feet or yards farther down the slope. The presence of a 
few fragments, or even of a single fragment, of iron ore in the wash is 
often sufficient to indicate the existence of an ore deposit several 
hundred yards away, which may be found by following up the wash 
and carefully watching its change in iron content. 
Where the laccolith and sedimentary rocks are found in direct con- 
tact it does not necessarily follow that ore will not be found deeper 
down along the contact. Replacements are usually irregular, and the 
source of the solutions is not from above. So far as the ores are fill- 
ings of fissures developed around the periphery of the andesite by its 
crystallization, it is easily conceivable that these openings would be 
irregular, both horizontally and vertically, and that where the dip of 
the laccolith contact is low, gravity might close them altogether, so 
that the appearance or nonappearance of ore on the erosion surface 
would be determined merely by the extent to which the erosion sur- 
face had cut down. 
In underground exploration the fact should not be lost sight of 
that the deposit may show the same range of irregularity with depth 
that it does on the erosion surface; in other words, that all the factors 
known to have determined the peculiarities and shapes of deposits 
on the erosion surface, which are discussed in this paper, should be 
taken account of in the underground exploration. 
