Geology of tJic St. Croix Dalles. — Bcrkey. 147 
quartz and chlorite. The finer material of the tuff is at the 
same time the more angular. It is altered chiefly to epidote 
and quartz. Large, well rounded grains are in relatively 
small amount. 
This is one of the few localities noted in the geological 
literature of the Lake Superior district where a well defined 
tufT derived from volvanic ash occurs. References made to 
similar accumulations on Michipicotin island by Selwyn/'' and 
at Duluth by Winchell and Grantt are the only descriptions 
with which the author is acquainted. 
Alteration Processes and Proelucts. Quartz, epidote, chlorite, 
calcite, orthoclase, hematite, magnetite, hornblende, actino- 
lite and copper are the usual secondary minerals of the dia- 
base. Quartz, epidote and chlorite are everywhere abund- 
ant. Those portions of the rock which have altered largely 
to quartz and epidote are the most firm and indestructible 
varieties. Many small veins are filled with a fine grained mix- 
ture of these two minerals which sometimes carry native cop- 
per in considerable amount. Quartz veins and cavities filled. 
or partially filled, with quartz are common. Epidote is some- 
times well crystallized in these cavities in small individuals. 
In many instances secondary orthoclase is associated with 
these occurrences. A fibrous quartz vein filling is also found. 
probably a pseudomorph after other secondary minerals. The 
quartz fillings of amygdules are in certain localities highly col- 
ored by ferric oxide distributed throughout the quartz grains 
in beautiful dendritic aggregates. Many amygdules are wholly 
filled with chlorite. In others epidote is associated with the 
chlorite in varying proportions to a complete replacement. 
The amount of secondary quartz is also found considerably 
more abundant with the epidote than with the chlorite, and 
in places it is a substitute for -both of these minerals. Chlorite 
fills the greater number of the smaller pseudo-amygdules and 
is the common secondary product derived from plagioclase 
and augite. Calcite occurs in amygdules and is sparingly 
distributed through the diabase of several localities. ^lag- 
netite and hematite are abundant secondary products. The 
rock takes on a dense black color due to their presence, while 
*Science, vol. I, 1893, pp. 11, 221. 
tAmer. Geol., vol. XVIII, Oct. 1896. pp. 211-213. 
