158 The American ficohxjist. September, 1893 
resulting tonus of importance are sea dill's and beaches, and 
cut terraces iVt't by the recession of the water. 
The fourth division is glacial erosion. To this class belong 
valleys made or modified by ice, rock basins, roches mouton- 
in'rs, hills rounded by glacial action, and plains leveled by the 
same means. 
I have placed all special deformation under the head of con- 
structive action, and the objection may be raised that this is 
an arbitrary classification. Strictly speaking, elevation is 
constructive and depression is destructive, but I am not aware 
that any actual forms of importance result from depression 
alone. l\' such forms exist, they would naturally constitute a 
fifth division under constructive action. Apparent depressions 
may really be due to elevations such as the synclinal depres- 
sion between two anticlinal folds. Depression sometimes takes 
place along one side of a fault, but it is usually, if not always, 
accompanied by an elevation of the other side, and, as far as 
1 know, all fault scarps are due chiefly to the latter action and 
so may be placed among- constructive forms. 
The preceding categories include all terrestrial topographic 
forms of importance resulting from inorganic agencies. The 
action of vegetable life might be classed as a distinct con- 
structive process but the forms resulting are not of great 
prominence. 
In polar regions, such as Greenland, ice covers the surface 
to the depth of several thousand feet and makes all the topo- 
graphic forms which the country possesses. In such cases the 
ice is to be regarded as a permanent stratum of the earths 
crust, subject to the same processes of deformation and 
erosion that affect other rock masses and yielding forms 
referable to the same divisions. 
The following table gives concisely the different processes 
with the resulting forms, and constitutes a summary of the 
plan of classification proposed in this paper: 
CAUSES. RESULTING FORMS. 
CONSTRUCTION. 
Volcanic Deposition. Eruptive mountains, craters, cinder 
and tuff cones and fields, lava flows 
and plains. 
