GEOLOGY: B. WILLIS 
381 
ficial depressions; (2) the surfaces of the Kghter interdiscs, riding high 
and covered with the sediments of epicontinental seas, or exhibiting an 
approximately vertical foliation where denuded to metamorphic rocks; 
(3) the marginal zones of discs and interdiscs, characterized by ex- 
trusion of igneous rocks and orogenic disturbances. Where a broad 
Hght mass or structural element lies adjacent to a broad heavy element, 
as along the Pacific Coast of North America, the zone of marginal 
deformation may be very wide. Where, on the contrary, two heavy 
elements are nearly tangent, the zone of disturbance may be exceedingly 
narrow, as the Isthmus of Panama. 
The hypothesis of discoidal structure may best be tested by the facts 
of the earth's major features and their relations to igneous intrusion 
and extrusion, to regional, periodic diastrophism, and to orogeny. 
Certain lines of investigation are suggested in what follows. 
Are igneous rocks distributed around deeps, as would be required by the 
discoidal hypothesis? — ^Let it be assumed that magmas originate, as 
postulated by Chamberlih , through local fusion in the outer few hundred 
kilometers of the lithosphere. Molten material gathering in the foliae 
beneath a disc would be guided by the foHation toward the surface and 
would appear in or near the outcrop of the margin of the disc. By 
postulate any disc is indicated on the surface by an oceanic deep or a 
continental basin. The relation of volcanoes and also of geologically 
recent batholiths to oceanic margins has long been recognized. As 
bathymetric observations accumulate a more detailed relation to in- 
dividual deeps is noticeable. The most conspicuous examples are the 
Windward Islands, sometimes called the "Necklace of the Caribbean," 
and the Alaskan, Aleutian, and Japanese volcanic chain. 
Does diastrophism exhibit that periodicity with reference to the larger 
deeps which we should expect if each deep constitutes a structural unit? — 
It is well established that the opposite coasts of the North Atlantic 
have periodically suffered similar and contemporaneous deformation, 
and the same is true of the encircling lands around the Pacific Ocean. 
Have the diastrophic activities of separate great deeps been non-con- 
temporaneous to the extent which would give to each an independent char- 
acter peculiar to itself? — That the diastrophic activities of the Atlantic 
and Pacific basins are incommensurate on the geologic time scale is a 
fact which constitutes the gravest difficulty in the assumption of 
universal periodicity. If diastrophism be due to an ultimate cause, 
operating periodically upon the inner mass of the earth, such as the 
accumulation of internal heat, an explanation of the diverse periodicity 
of different oceanic basins must be sought through the mechanism 
