JAMES DWIGHT DANA. 607 
and Epochs has been acquired. Other manuals speak of forma- 
tions, systems and €tages, of series and groups; rocks being clas- 
sified as if they were distinguished by some qualities of their 
own. Itis from Dana that we have learned to classify geological 
formations in relation to the stages of progress in the building 
of the continents and its local structural features, and to regard 
rocks as not simply aggregates of mineral matter, but as geological 
formations bearing a definite relationship to the progress in the 
history of the earth, and hence as belonging to, and to be defined 
as of a particular period or epoch. In the first edition of his 
Manual in 1862 the author wrote: 
“It has been the author’s aim to present for study, not a 
series of rocks with their dead fossils, but the successive phases 
in the history of the earth,—dits continents, seas, climates, life 
and the various operations of progress.’’* 
The grand outlines of Dana’s system of the earth’s develop- 
ment are given in a few sentences in his article ‘‘On the Plan of 
Development in the Geological History of North America,” first 
published in the Asmerican Journal of Science in 1856.7 
“What then is the principle,’ he wrote, ‘of development 
through which these grand results in the earth’s structure and 
features have been brought about ? We detect a plan of progress 
in the developing germ; we trace out the spot which is first 
defined, and thence follow the evolution in different lines to the 
completed result: may we similarly search out the philosophy of 
the earth’s progress? The organizing agencies in the sphere 
are, 1) Chemical combination and crystallization. 2) Heat, in 
vaporization, fusion and expansion, with the correlate force of 
contraction which has been in increasing action from the time the 
globe began to be a cooling globe. 3) The external physical 
agencies, preéminently water and the atmosphere, chiseling and 
*Manual of Geology: treating of the principles of the science with special 
reference to American Geological History, for the use of colleges, academies and 
schools of science, by JAMES D. Dana, pp. xvi.-++ 798, illustrated by a chart of 
the world, and over one thousand figures, mostly from American sources: Philadelphia 
and London, 1863. 
?Am. Journ. Sci. IJ., Vol. XXII., p. 339. 
