STRUCTURE AND SURFACE FEATURES 419 
4. BASINS 
We have now considered the origin of the more important 
surface features—mountains, plains, plateaus, and valieys— 
and briefly indicated to what extent the varying character of 
rocks and rock-structures has influenced their development. 
There is yet another interesting class of land-forms deserving 
of attention by the student of structural geology. We refer 
to the larger and smaller depressions of the surface, the 
_ majority of which are now or have formerly been occupied by 
water. Like other superficial features, Baszvs are of various 
origin, some being the result of crustal deformation, others 
owing their formation to epigene action, while yet others are 
due to both. 
(z) Tectonic Basins.—Most of the larger lakes and 
many inland seas occupy basins which have come into 
existence during earth-movements. In some cases these 
depressions are geosynclines—the result of a local sagging 
or subsidence of the crust, not necessarily accompanied by 
fracture and dislocation. In other cases, subsidence has taken 
place along lines of faulting and disturbance. Some basins, 
again, would seem to have come into existence between 
contiguous high grounds undergoing elevation. The great 
lake-basins of Russia and North America (Onega, Ladoga, 
Superior, Huron, Michigan, etc.), and the extensive Aralo- 
Caspian depression, with its numerous sheets of water and 
desiccated basins, are essentially geosynclinal troughs. The 
Dead Sea and the lakes of Equatorial Africa, on the other 
hand, occupy depressions caused by fracture and displacement. 
It is worthy of note that Tectonic basins are not confined 
to any particular latitude. A considerable number, apparently 
the majority, occur in relatively dry and rainless regions— 
both at low and high levels. On the other hand, not a 
few are met with in temperate and well-watered lands, of 
which the large Russian and North American lakes are the 
most notable examples. 
(4) Volcanic Basins.—The most typical basins of this 
class mark the sites of extinct volcanoes. Many lakes, for 
example, occupy the cup-shaped depressions of volcanic 
cones; or the deep concavities in the surface of the land 

