Geology of Hic St. Croix Dalles. — Bcrkey. 1 5 1 
Azurite. The blue carbonate has been noted in association 
with malachite and dolomite. Other copper minerals have 
been reported from this locality but have not been encountered 
during this investigation, and visits to the sites of old mines 
have not usually shown any traces of the minerals for which 
they were worked. 
Orthoclase. Secondar}- feldspar is rather well developed 
in cavities of the rocks where there has been considerable 
alteration. It is a flesh red mineral altering readily to quartz 
and frequently exhibiting crystal outlines. It is associated 
chiefly with quartz and epidote. 
Lab^'ddorite . Both original and secondary feldspars are 
])resent in the diabase rocks. The original representatives of 
this group all belong to the plagioclase division near "labra- 
dorite." There is no essential dilTerence between the large 
phenocrysts of the porphyritic phases and the smaller indi- 
viduals of the second generation in the ground mass. All 
the feldspars show more or less alteration to kaolin, chlorite, 
epidote and (piartz, and in many cases nothing remains but 
the outlines of these pseudomorphs to indicate the character 
and position of the original constituent. 
Augite. This mineral is prominent in the fresh erupti\e 
Tock. It is especially well-developed in the lustre-mottled 
variety, where crystals of this mineral serve as the hosts for 
numerous plagioclases giving a typical ophitic structure. 
Hornblende . Hornblende is not common. It occurs how- 
ever, as a secondary mineral in a few sections. The speci- 
mens already noted indicating a replacement of fibrous born- 
blende by quartz are the most interesting. 
Aetitiolite. In many sections cut from the more highly 
altered rocks, and especially from those carrying a consider- 
able amount of secondary (piartz. is a fine fibrous mineral 
which is believed to be actinolite. Its' fine, hair-like fibers, 
usually crystallized in radiating bundles, penetrate the (juartz 
grains in great j^rofusion. This mineral has been noted chiefly 
in the bed of volcanic tuff and in the brick- red blotches occur- 
ring in the diabase at the elbow of the river on the Wisconsin 
side. This is supposed to be the nu'neral referred to by Kloos 
^nd Streng as apatite needles. 
Muscovite. A lis>"ht colored mica identified as muscoxite 
