RECENT ADVANCES IN GEOLOGY. 465 
lakes of high northern latitudes have proved of equal inter- 
est. In the Swedish lakes, Wetersee and Wenersee, have 
lately been discovered crustacea which, though differing from 
those now living in the sea, are clearly related to marine 
forms of a northern and even Arctic character. Thus have 
been found the Mysis relicta, whose congeners live altogether 
in the sea, and those resembling the species in the most 
northern latitudes ; the Que loricatus thus far found 
only in the Arctic Ocean, Baffin's Day, Greenland, and Spitz- 
bergen; the Zdothea entomon, in the Arctic Ocean and the 
Baltic Sea; and the Pontoporcia affinis, still found in the 
Baltic, but whose related species occur in the Greenland 
seas. These lakes are three hundred feet above the sea- 
level; but these results show that at no remote day they 
communicated with the ocean, and were originally tenanted 
by a marine fauna of an Arctic type. As these waters be- 
came first brackish and then“ fresh, most of the forms died 
out during the transition, leaving in the depths a few crusta- 
cea which i correspond in part’ to the species in the Baltic, and 
in part to those of the Arctic Ocean. 
Within the past year Dr. Stimpson has. obtained results 
equally interesting, from dredgings brought up from the 
deeper parts of Lake Michigan. The lake-level is five hun- 
dred and eighty-three feet above the ocean, and the greatest 
depths extend below that line. At the depth of sixty 
fathoms he obtained à Mysis which, although not specifically 
identical with the Swedish form, is closely allied, and its 
occurrence authorizes us to draw the same conclusions as to 
the marine character in former times of the Great Lakes, 
which the Swedish physicists . have arrived at as to the 
former condition of their own. 
Much discussion has been had in former years, and even 
in this Association, as to the nature of these lake waters dur- 
ing the Glacial Age. It is well known that on the borders 
of Lake Champlain, and at intervals along the St. Lawrence 
from Quebec to Kingston, and up the Dita the terraces 
59 
AMER. NATURALIST, VOL. IV. 59 
