230 On the Surface Geology of the Basin of the 



fore we can be said to have solved all the problems it involves. 

 There are. however, certain facts connected with the structure 

 of the lake basins, and some ded actions from these facts, which 

 may be regarded as steps already taken toward the full under- 

 standing of the subject. These facts and deductions are briefly 

 as follows : — 



1-t. 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 Bomewhat modified by this agent. 



I'd. Lake Huron, Lake Michigan, Lake Erie, and Lake 

 Ontario are excavated basins, wrought out of once continuous 

 Bheeta 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 surfaces — are all 

 furrowed by glacial grooves, which are parallel with the major 

 of the lake. 



All our great lakes are probably very ancient, as since the 

 ■ of th<- Devonian period the area they occupy has never 

 u submerged beneath the ocean, and their formation may 

 have begun during the Coal Measure epoch. 



The Laurentian belt, which Btretcbes from Labrador to the 

 Lake of the Woods, and thence northward to the Arctic Sea, 

 forms tin- oldest known portion of die earth's surface. The 



shore- <>!' this ancient < tiueiit, then high and mountainous, 



were washed by the Silurian sea, where the debris of the land 



was deposited in strata that subsequently rose to the Burface, 

 and formed a broad low margin to the central mountain belt, 

 just a- the Cretaceous and Tertiary strata Hank the Allegha- 

 nies in the Southern Stat< 



hi ih- lapse of countle .all the mountain peaks and 



