January 12, 1894 | 
SCIENCE : 
PusiisHep By N. D. C. HODGES, 874 Broapway, NEw York. 
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VOLCANIC ROCKS IN THE KEEWATIN OF MIN- 
NESOTA.* 
BY ULYSSES SHERMAN GRANT, MINNEAPOLIS, MINN. 
Tar the Keewatin rocks northwest of Lake Superior 
are toa considerable extent composed of volcanic (effusive) 
material has been stated already by G. M. Dawson,’ A. C. 
Lawson’ and N. H. Winchell’. Although the material of 
much of the Keewatin in Minnesota has been assumed to 
be volcanic tuff and finely divided, water-deposited ash, 
the actual number of places where the rocks have been 
shown to be composed of such volcanic matter is very 
small. M. E. Wadsworth has described a few sections of 
fragmental volcanic rocks,—-porodites,-— and N. H. Win- 
chell has given an account vf an agglomerate from Ely.° 
Aside from these the writer knows of no descriptions of 
rocks from the Keewatin of Minnesota that are clearly 
shown to be of volcanic origin. 
In the neighborhood of Kekequabic Lake, in the north- 
ern part of Lake County, a rock has been found which 
proves to bea volcanic fragmental. It isin the midst of 
thick Keewatin strata,—argillites, graywackes, conglome- 
rates, green schists, etc..—which form the eastern exten- 
sion of the Vermilion iron range. The rock under dis- 
cussion varies much in general appearance, but is usually 
of a greenish color and very compact and tough. The 
groundmass is aphanitic and in it are numerous, lighter 
blotches and changes of color, between the blotches, 
and sometimes in them are black crystals of horn- 
blende; pyrite is also quite common. In certain 
places rounded and subangular pieces of quartz 
and argillite are embraced inthe rock. Parallel color- 
bandings resembling sedimentary laminz also occur, occa- 
sionally quite abundently, but usually the rock shows no 
structural planes of any kind, nor any schistose or slaty 
cleavage. In thin sections the fragmental character of 
the rock is easily discernible. The fragments are usually 
angular, and their original nature is not always evident, 
owing to alteration and to the development of secondary 
minerals, but it seems that a porphyrite forms most of 
these fragments. The groundmass of the rock is largely 
fibrous hornblende. ‘There are also areas of secondary 
hornblende, both in the groundmass and in the fragments, 
filling in old crystal outlines; what crystals originally fll- 
ed these places is not clear, but it is probable that they 
1 Published by pebatssion of the State Geologist of Minnesota. 
2 British N. A. Boundary Commission, 1875. 
, eet Se panage) wok iii, pt. i, 1888. 
xeol. and Nat. Hist. Sury. of Minn., 15th Ann. Rept., 1887; 16th 
Rept , 1888; Bull. No, 6, 1891. oie Beater aote Sans 
5 Ibid.; Bull. No. 2, 1887. 
® Amer. Geol., vol. ix, pp. 359-368, June, 1892. 
© 
SCIENCE. 
- well developed. 
17 
were pyroxene phenocrysts. The rock thus appears tobe 
a volcanic tuff completely solidified and more or less al- 
tered. 
Closely associated with this volcanic tuff, and grading 
into it, are some peculiar green schists, which, like the 
groundmass of the former, are made up almost entirely of 
hornblende. That these schists are composed of water- 
deposited material is clearly shown by numerous lami- 
ne, frequently running at an angle with the schistose 
cleavage, and in places by rounded quartz pebbles ar- 
ranged in parallel lines. Under the microscope a sec- 
tion from these schists is seen to consist of closely crowd- 
ed, green hornblende crystals embedded in a fine, fibrous 
groundmass also made up of hornblende. The crystals 
are, in short, stout prisms, averaging about a quarter of 
a millimetre in length. They are commonly, not com- 
pletely idiomorphic; the prismatic planes are very gener- 
ally quite distinct, but the terminal faces are not so often 
The ends of the crystals often show 
fringes or fibrous prolongations running out into the 
groundmass. At times these fringes are sharply marked 
off from the crystal proper, being of alighter color, but 
the fringe on each crystal is optically continuous with it. 
These fringes closely resemble the secondary enlarge- 
ments of hornblende grains and crystals described by C. 
R. Van Hise’ and are probably of a similar nature. As 
to just what was the nature of the sediment which formed 
these green schists, it is impossible to decide, but there 
are several reasons for regarding it as chiefly finely divid- 
ed,water deposited, volcanic ash now entirely recrystallized. 
In the immediate neighborhood of the tuff and green 
schists is a small area ofa fine-grained, purple, porphy- 
ritic rock, which proves to be a hornblende porphyrite. 
This has distinct, elongated phenocrysts of brownish 
hornblende imbedded in a completely crystalline ground- 
mass of interlacing feldspar laths; in places this ground- 
mass becomes almost granular in structure. From its 
known relations to the surrounding rocks, the complete- 
ness of its crystallization and the absence of structures 
common to effusive rocks, this porphyrite appears to have 
never reached the surface, although such a degree of 
crystallization might possibly have been attained in the 
centre of a very ‘thick flow. 
It seems probable, then, that in Keewatin (Lower 
Huronian, as that term is used by the United States Geo- 
logical Survey) time a volcano existed in this locality, 
that it furnished the deposits of tuff and ash described 
above, that its lava was of about the composition of horn- 
blende porphyrite, and that the present known mass of 
porphyrite represents a part of the igneous magma which 
solidified below the surface. 
A more complete account of these volcanic rocks and 
the surrounding rocks, especially of an interesting augite 
soda-granite, concerning which a preliminary note has 
already been published,* will appear in the forthcoming 
twenty-first annual report of the Geological and Natural 
History Survey of Minnesota. 
—In view of the present interest in German politics, 
social and political life, and educational affairs, the new 
work, ‘‘Germany and the Germans,” by William Har- 
butt Dawson, is timely. Mr. Dawson, who is well 
known as the author of ‘‘German Socialism and Ferdi- 
nand Lassalle,” and ‘‘Prince Bismarck and State Social- 
ism,” has made a close study of German life and insti- 
tutions at the present day, and the results of his obser- 
vations are set forth in an interesting manner. ‘‘Ger- 
many and the Germans” will be published immediately 
by D. Appleton & Co. 
7 Amer. Jour. Sci., III, vol. xxx, pp. 231-235, Sept, 1885. 
8 Amer. Geol., vol. xi, pp. 383-388, June, 1893. 
