428 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
expand into bodies of considerable size, and when the ocher is removed 
rooms 6 to 10 feet in diameter are sometimes left, connected by nar- 
row, winding passages. The mining of the ocher has left the point of 
the ridge completely honeycombed with these irregular passages and 
rooms. 
The contact between the ocher and the inclosing quartzite is never 
sharp and distinct, but always shows a more or less gradual transi- 
tion from the hard, vitreous quartzite to the soft ore which may be 
easily crushed between the fingers. The quartzite first becomes 
stained a light yellow and loses its compact, close-grained texture. 
This phase passes into a second, in which the rock is perceptibly 
porous, having a rough fracture and a harsh "feel," and containing 
enough ocher to soil the fingers. In the next phase the ocher prepon- 
derates, but is held together by a more or less continuous skeleton of 
silica, although it can be readily removed with a pick. The final 
stage in the transition is the soft yellow ocher, filling the veins, which 
crumbles on drying and contains only a small proportion of silica in 
the form of sand grains. 
The intermediate zone between the pure ocher and the quartzite is 
usually a few inches in thickness, although it may be several feet 
between the extremes, and, on the other hand, sometimes only a 
fraction of an inch. When the transition rock is examined under 
a microscope the character of the transition can be seen even more 
clearly. The more compact portions, which are only slightly stained 
with iron, are seen to be composed of a transparent gronndmass, 
threaded with minute cavities which penetrate the rock in all direc- 
tions and contain a tine dendritic growth of iron oxide. The latter 
occurs only rarely in isolated grains, but generally in clusters of 
minute grains or fibers attached to one another and branching irregu- 
larly from a central stem. They have no trace of crystal form. 
Passing toward the ore body, these minute passages become larger and 
increase in frequency, until only a finely branching siliceous skeleton 
remains, the greater part of the rock having been replaced by the 
iron oxide. Under polarized light the transparent gronndmass is 
broken up into an aggregate of small quartz grains penetrated in all 
directions by the iron oxide. The latter does not lie between the indi- 
vidual grains, but passes through them as though the groundmass were 
quite homogeneous. The process of replacement is never complete, 
for all the ocher contains more or less sand. When this is washed 
clean from the iron oxide it is found to differ from ordinary sand 
grains in having extremely irregular outlines. This sand, as might 
be anticipated from the microscopic structure of the slightly altered 
quartzite, is evidently composed, not of the original grains of the 
rock, but of detached portions of the irregular siliceous skeleton 
which in the intermediate stages of replacement holds the iron oxide 
in its cavities. Aside from the silica the ocher as mined contains 
