Geology of tJie St. Croix Dalles. — Berkey. 149 
removal of soluble substances are especially favorable. An 
intermediate stage common in varying degrees to each line 
of alteration is that of chloritization. There seems to be no 
variety of this rock wholly free from this last named product. 
Chapter III. Minerals. 
Gold. An assay of the pyritiferous shales at Taylor's 
Falls shows traces of gold, but beyond this no evidence of the 
precious metal was found in any of the rocks of this area. 
Copper. Native copper occurs in small quantity in the 
epidotic portions of the diabase flows. A thin section cut 
from a rock specimen taken from an epidote vein, Sec. i, T. 
34 N., R. 19 W., shows numerous grains of copper scattered 
throughout the slide. The matrix is a finely crystalline in- 
termixture of secondary quartz and epidote. Copper is also 
found occasionally in the glacial drift. 
Pyrite. This mineral occurs sparingly in the igneous 
rocks. It is most readily obtained in the railroad cut at Tay- 
lor's Falls and at a similar cut on the "Soo" road, two miles 
north of Dresser Junction. Pyrite occurs in great abund- 
ance, however, in the Lower Dresbach shales. The finest 
specimens were obtained in the small ravine below the card- 
ing mill in Taylor's Falls. In portions of the shale at this 
place small rounded concretions of secondary pyrite the size 
of a pin head constitute fully one-fourth of the bulk of the 
rock. Forms of brachiopod shells are also preserved by the 
pyrite. The plentiful yellow and white efflorescences formed 
on the exposed surfaces of these shales are no doubt chiefly 
the result of decomposition of the pyrite. 
Quartz. This mineral is crystallized sparingly in the 
larger cavities of the diabase. It occurs as a coarsely crystal- 
line filling in amygdules and in veins. In the form of more 
or less rounded grains it constitutes the bulk of the sedimen- 
tary strata of the area. A cryptocrystalline variety is noted in 
certain of the sections of volcanic ash. 
Magnetite. Primary and secondary magnetite is abundant 
in most varieties of the igneous rocks of the area. As a pri- 
mary constituent it occurs in grains of more or less regular 
outline imbedded in the diabase. As a secondary constituent 
it occurs in irregular aggregates and branching spear-like 
