Geologn of the Sf. Croix Dalles. — Berkey. 381 
foot of each step. In some instances also there is a zone of 
greater porousness at the top of each step than is usual in 
other parts, and greater local differences are developed in the 
rock. Greater hardness in places separated by areas more 
readily destroyed has given rise on one of these benches to one 
of the most magnificent exhibitions of pot-hole erosion any- 
where to be found: In at least two places a decidedly schist- 
ose character has been developed at the immediate line of sep- 
aration. In a good hand specimen the bands and veins show a 
considerable crumpling of the la^^er. 
On the bench e;tst of the river below the toll bridge the so- 
called flowage structure is extensively displaj^ed. On certain 
portions of the bench above the public school building and at 
least at one point on the east side of the river a distinctly" 
brecciated structure has been preserved. This is offered as 
an addition to the evidence of the complete independence 
of these successive steps and a |)roof of the theory of separate 
surface flows. 
In this zone occurs a limited development of the tough blue 
diabase and the still more limited tufaceous rock which has 
been found at three different horizons at Taylor's Falls. (See 
chapter on the lithology of the igneous rocks.) In this zone 
secondary quartz is so abundantly formed in places as to 
make up almost one-half of the rock although secondary 
quartz is by no means confined to this plane. 
Quartz veins, secondary feldspar, grains and threads of 
metallic copper occur. In no other corresponding portions of 
the flows are there any correspondingly great variations. At 
one point a quartz segregation, filling one of these lateral 
separation planes, has been worked for copper, but with no 
success. 
' The set of joints and columns is usually interrupted at these 
separation planes. Below this zone in every case the typical 
flow shows a considerable uniformity of adjacent areas, a 
stronger tendenc}' to the formation of joints and columns, a 
very dark color and a typical diabasic texture. Lustre, mottling 
porphyritic development and chhuitic alteration products 
aie also more common. 
The thickness of the sevcial fiows as thus separated varies 
between 30 and 00 feet. Tiiat thrv mav include other thinner 
