CAUSE OF EARTHQUAKES—HOBBS 273 
its hinge, the hinge line for the Laurentian region taking an 
average direction in the neighborhood of the Great Lakes of 
about 15° to the north of west. The case is, however, not quite 
so simple as this, for the main hinge line of the region has itself 
migrated northward since the beginning of the uptilt, and_ sec- 
ondary hinge lines within the trap-door itself appear also to have 
functioned. One may liken the complex movement in its main 
SS 
Tee 
mma 
a 
bie 
ces 2 
| peat a 
Fic. 4.—Diagram to show the nature of the Laurentian region as the 
continental glacier retired northward 
lines to that of a trap-door made up of several planks all parallel 
to the main hinge and each hinged to its neighbors, all hinges 
being or having been in action. (Fig. 4.) 
We are now chiefly concerned, however, with the evidence that 
uptilt of the land is still going on, and may therefore be responsible 
for the earthquakes of the region. The earliest clear recognition 
of such present-day uptilt was made by a land surveyor of Wisconsin, 
