NUMAKAGON LAKE TO THE ST. CROIX RIVER. Zap 
ures are altogether of the beds of the Lower Division. The relative positions 
of the exposures, and the bedding directions observed at them, show that 
immediately west of the district last described, or in ranges 6 and 7 west, 
a rapid flattening of the northern dip takes place, and a proportionately 
great widening of the belt of country occupied by the formation. In range 
7 west this width is as much as 9 miles, the dip flattening to 35° in the 
lowermost belts and to 25° in the upper belts. Farther west the dips flatten 
still more, getting as low as 10° to 15°, and the surface width becomes as 
much as 12 to 15 miles. 
As to kinds of rocks, the exposures are sufficiently frequent to show 
that the constitution of the Lower Division of the series is the same in this 
region as farther east. The greatly predominant rocks are the usual plainly 
bedded fine-grained diabases with their amygdaloids. Melapliyrs or olivine- 
diabases are rarer, but occur somewhat frequently. Interleaved porphyry- 
conglomerate and sandstones have been observed at a number of points, 
while massive quartz-porphyry is occasionally to be seen. No coarse gab- 
bro has been observed anywhere between Numakagon Lake and the Saint 
Croix River. A 
A rather unusual phase of diabase is the diabase-porphyry of the ledges 
in sections 26 and 27 of T. 36, R.16 W. These rocks, “in a greenish-gray, 
fine-grained matrix have a greater or less amount of red feldspar in porphy- 
ritic crystals, one-thirtieth and one-eighth inch in diameter.” The specific 
gravity is 2.90.° Under the microscope the porphyritic crystals prove 
to be a somewhat altered triclinic feldspar, while the groundmass is chiefly 
made up of smaller plagioclases. Intermingled with these are fibrous, 
greenish, feebly dichroic or non-dichroic particles, which prove to be altered 
(uralitic and viriditic) diallage; epidote, in quite abundant grains; titanic 
iron largely altered to a white substance ; and pseud-amygdaloidal chlo- 
rite. Closely related to this, and evidently at about the same horizon, is 
the dark greenish-gray porphyritic rock so largely exposed at the Dalles 
of the Saint Croix, in T. 34, R. 19 W2 In this rock porphyritic brown 
‘See Geology of Wisconsin, Vol. III, pp. 41,42; 46-48; 391-395 3 399-428, for lithological deserip- 
tions and complete local details. . 
*R. Pumpelly, Geology of Wisconsin, Vol. III, p. 41. 
* Geology of Wisconsin, Vol. III; also A. Streng and J. H. Kloos in Leonhard u. Geinitz Jahrbuch 
fiir Mineralogie. 1877. 
