W. J. McGee—Dynamical Geology. 489 
Fic, 4. 
Fic. 3. 1, front view of a weathered fragment of the apical extremity of Actinoceras 
Bigsbyi 2; showing (f) the large foramen and the row of minute foramina 
situated at f’, the most elevated part of the siphuncular segments. Natural size. 
From the Black River Formation (?), Igloolik Island, Arctic America. 
Fic. 4. View of part of a much eroded fragment of an Actinoceras, consisting of the 
internal cast of some of the septal chambers, and the siphuncular segment with 
its foramen (f). Three-quarters natural size. Fre. 3.—2, side view of part of 
the same specimen, showing the lateral position of the large foramen (/) in 
relation to the apex. Natural size. From the Great Slave Lake, British North 
America. 
IV.—Some Derinitions ry DynamicaL GEoLoey. 
By W. J. McGee; 
Of the United States Geological Survey, Potomac Division, Washington, D.C. 
N view of the active discussion of the problems of earth-move- 
ment and mountain-growth now current, certain fundamental 
definitions, growing out of the discrimination of processes commonly 
confounded but really distinct, seem to be timely. 
The various processes with which the geologist has to deal fall 
naturally into two principal and antagonistic categories and five 
subordinate and supplemental categories; and each category, great 
and small, comprises two antagonistic classes of movements or 
agencies. 
The initial geologic movements (so far as can be inferred from 
the present condition of the rocks of the earth) were distortion or 
