The Plan of the Earth and its Causes. — Gregory. 141 
ean trough between ; and far to the south we liave the old 
plateau of South Africa. 
Let us now go 120" westward to the American zone. It 
begins with another block of old Archean rocks, forming what 
Suess has called the "Canadian schild." It occupies Canada, 
Labrador, and most of Hudson bay and Baffin's Land, and un- 
derlies Greenland. Bands of marine deposits surround it, but 
it has perhaps never been itself below sealevel ; its geological 
age, at any rate, is enormous. South of the North American 
coign we have again a pair of east-west mountain chains, form- 
ing the highlands of Cuba and Venezuela, separated by the 
Caribbean trough. This zone also ends southwards in an old 
plateau resting on Archean rocks. 
The third meridional zone repeats the same characters. It 
begins with a block of Archean rocks, of which we may speak 
as the "]\Ianchurian coign." South of this coign are the east 
and west ridges of ^Malaysia and the depressions parallel to 
them ; and south of that, again, we have the Archean plateau 
of Australia. 
The three main land axes of the world have remarkable 
resemblances in structure, and they present three equidistant 
blocks of great stability at the three tetrahedral corners. We 
may, therefore, speak of the "schild" as the three northern 
coigns or corner-stones of the earth. 
The existence of these massive coigns* at the three tetra- 
hedral corners has produced one point of divergence in the 
earth-plan from the geometrical figure of the tetrahedron. The 
existence of three such broad massive blocks naturally 
strengthens the line between them ; and, as we have seen, the 
main divide in the northern hemisphere runs from coign to 
coign. The tetrahedral edges would naturally be lines of weak- 
ness and of movement ; but in the northern hennisphere, the 
horizontal lines of yielding are deflected southward by the 
stability of the band supported by the earth's three northern 
coigns. Hence the great band of disturbances is subtropical, 
and runs from the Caribbean to the Mediterranean, across the 
Persian gulf and the Malaysian archipelago. 
* This suggestion of the word "coign" for "corner" I owe to Mr. L. Fletch- 
er, to whom I am indebted for much helpful advice. The term is suitable, as it 
is used for a printer's wedge as well as for the corner-stone of a house. 
