606 THE JOURNAL OF GEOLOGY. 
in elevation of the continents, — (/) the separation of the history 
of the earth into ages by the revolutions at the climaxes in the 
contraction, when strain and tension came to exceed strength 
and resistance, and resulted in folding and faulting and local 
disturbances, and were marked by the greater or less extermina- 
tion of life, followed by repeopling by, and the modification of the 
successors, — (2) the surface shaping of the continents by ice and 
water action also influenced by oscillation of level of the con- 
tinents; and all of these various factors taking a part in produc- 
ing the present complex condition of the earth’s surface. 
The earth as a whole was the unit which was before his mind 
as he constructed this system of geology. As he traced its history 
he saw in the successive events of geology the marks of the 
gradual development of a vaporous, then incandescent, and finally 
hardened, contracting, cooling globe. Others had spoken of 
geology as a history; but he appears to have been the first to 
write a manual of geology in English based on this idea. “In 
history,’ he commented, ‘the phases of every age are deeply 
rooted in the preceding, and intimately dependent on the whole 
past. There is a literal unfolding of events as time moves on, 
and this is eminently true of geology.” Hence he began his 
geology with the beginnings, and followed the course of the 
history of the earth onward. 
Again, to Dana the means of measuring the sequence of events 
was the succession of fossils. ‘Geology is not simply the science 
of rocks, for rocks are but incidents in the earth’s history, and 
may or may not have been the same in distant places. It has its 
more exalted end, — even the study of the progress of life from 
its earliest dawn to the appearance of man; and instead of saying 
that fossils are of use to determine rocks, we should rather say 
that the rocks are of use for the display of the succession of fossils. 
Both statements are correct; but the latter is the fundamental 
truth in the science.” It was this idea which dominated in his 
classification of geological formations. 
American geologists are all aware that it is from the use of 
Dana’s system that the habit of speaking of geological Periods 
