Geology of the St. Croix Dalles. — Berkey. 143 
reached that the rock could with equal correctness be called 
a "diabase." 
Local variations of the diabase. Although there is a 
general similarity in the different outcrops over the whole 
area, it frequently happens that very unlike varieties are 
formed within a few feet in the same exposure. In many cases 
also this difiference is not reducible to any known law of posi- 
tion. Porphyritic phases occur immediately adjacent to com- 
pact and uniform finely crystalline phases in the same bed 
and at the same level. This can be said also of many exam- 
ples of the amygdaloid and pseudo-amygdaloid. In general, 
however, the porphyritic varieties are most prominent in the 
upper flows,* while the ophitic character is best seen in the 
lower flows. These extremes of variation are noted below. 
Lustre-mottled Diabase. This variety forms the bases of 
nearly all of the descriptions of the rocks of this locality here- 
tofore published. It is the commonest phase of the compact 
uniformly crystalline rock. Its greatest development is in 
the vicinity of the Dalles, although not confined to that out- 
crop. On weathered surfaces it is pitted and brown-colored 
from the development of secondary products. On fresh frac- 
tures the rock is greenish-black and exhibits numerous spots 
over the surface which reflect light from a single cleavage 
plane. These spots are augite areas, and imbedded in them 
are many feldspar crystals, the two minerals producing a typi- 
cal ophitic structure. Other minerals are magnetite and sec- 
ondary chlorite, epidote, quartz, and kaolin. Not even in the 
freshest sections has there been found a grain of olivine or a 
fragment of original glassy matter. 
PorpJiyritic Diabase. At many localities the diabase 
shows a marked porphyritic development. The ])henocrysts 
are of plagioclase feldspar near labradorite in extinction, from 
gray to brick red in color, and in size reaching a length of two 
or three inches. The most persistent and extensive occur- 
rences of the porphyritic diabase are in the series of outcro])s 
in sections 13, 24 and 25, T. 34 N., R. 19 W., in the southern 
portion of Taylor's Falls village along lower Mill street, and 
in several localities on the Wisconsin side of the river. It is 
always limited in extent. A specimen collected in S. E. j S. 
*Owen: Geological Survey of Wis., Iowa and Minn., 1852, p. 164. 
