210 SURFACE GEOLOGY. 
ORIGIN or THE GREAT LakEs. — The question of the ori- 
gin of our lakes is one that requires more observation and 
study than have yet been given to it before we can be said to 
have solved all the problems it involves. There are, how- 
ever, certain facts connected with the structure of the lake 
basins, and some deductions from these facts, which may be 
regarded as steps already taken toward the full understanding 
of the subject. These facts and deductions are briefly as 
follows :— 
Ist. Lake Superior lies in a synclinal trough, and its mode 
of formation therefore hardly admits of question, though its 
sides are deeply scored with ice-marks, and its form and area 
may have been somewhat modified by this agent. 
2d. Lake Huron, Lake Michigan, Lake Erie, and Lake 
Ontario are excavated basins, wrought out of once contin- 
uous sheets of sedimentary strata by a mechanical agent, and 
that ice or water, or both. 
That they have been filled with ice, and that this ice 
formed great moving glaciers we may consider proved. The 
west end of Lake Erie may be said to be carved out of the 
Corniferous limestone by ice action; as its bottom and sides 
and islands—horizontal, vertical, and even overhanging sur- 
faces—are all furrowed by glacial grooves, which are par- 
allel with the major axis of the lake. 
All our great lakes are probably very ancients as since the 
close of idee Devonian period the area they occupy has never 
been submerged beneath the ocean, and their formation may 
have begun during the Coal Measure epoch. 
The Laurentian belt, which stretches from Labrador to the 
Lake of the Woods, and thence northward to the Arctic sea, 
forms the oldest known portion of the earth’s surface. The 
shores of this ancient continent, then high and mountainous, 
were washed by the Silurian sea, where the débris of the 
land was deposited in strata that subsequently rose to the 
surface, and formed a broad low margin to the central moun- 
tain belt, just as the Cretaceous and Tertiary strata flank the 
Alleghanies in the Southern States. 
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