Topographic Forms. — Perri/. 155 
cesses. In general it ma}- be said that all the geographic- 
features of the earth are due to elevation and depression. All 
land masses had first to be raised above the ocean level before 
other processes could give them their final characters and 
shape them into plains and mountains. In this way conti- 
nents and mountains were primarily formed. But the topo- 
graphic features treated in this paper are not such general 
features as continents. They are the special forms which the 
surface of the earth has locally assumed, and in their produc- 
tion the forces of deformation have played directly but an 
insignificant part. A portion of the earth is raised above the 
ocean by displacement, but its final character as a mountain 
range or mesa or prairie is given it by the constructive or de- 
structive action of particular agencies. That process which 
gave the final character is the one to which the form properly 
belongs, no matter what other causes may have affected it 
more remotely. For instance, a mesa produced by a lava flow 
protecting from erosion the softer strata beneath is properly 
a product of erosion and not of volcanic action, for the result 
would be the same if the lava flow had been replaced by a 
stratum of any other durable rock. The same is true of 
laccolites, dikes, and necks, exposed by the removal of sur- 
rounding strata, and which are here classed as products of 
degradation. Deformation may be considered an underlying 
cause of all surface modifications, but it will not be classed 
as an immediate agency except where its local action has pro- 
duced forms that have not been materially altered by other 
processes.* 
Constructive action may be classified under five divisions. 
The first is volcanic deposition, including all forms built up 
from ejected materials. This may be called the most import- 
ant constructive agency, for the chief products of its action, 
volcanic cones, are among the most prominent features of the 
earth's surface. To this division belong all volcanic moun- 
tains, craters, cinder and tuff cones and fields, and lava plains. 
*McGee, in his paper entitled ''A Classification of Geographic Forms 
by Genesis" (National Geog. Mag., Vol. I, No. li, takes the opposite 
ground with respect to orogenic and epeirogenic deformation, and classes 
mesas among volcanic products. He does not make construction and 
destruction the basis of his classification, but subdivides each of his 
main divisions, Vulcanism, Glaciation, etc., into constructive and de- 
structive action. 
