JAMES DWIGHT DANA. 609 
not only made a thorough study of the volcanoes in the islands 
of the Pacific and on the borders of the South American conti- 
nent, and Vesuvius and “tna in Italy (his first scientific paper 
as before noticed was a letter written from U. S. frigate ‘United 
States” in 1834 ‘On the condition of Vesuvius in July 1834”), 
but in his investigations of the many questions raised by these 
observations he also studied the surface of the moon,—and com- 
parison of the already cooled moon and its extinct craters with 
the present condition of the earth suggested the chief phenom- 
ena about which was later elaborated his theory of the earth’s 
development as a cooling, and necessarily contracting globe. 
This paper, read before the Association of American Geolo- 
gists and Scientists in September 1846, ‘‘On the Volcanoes of the 
Moon,” suggested the following: ‘‘Thoughts bearing on our own 
planet.” 1. If the earth was once a melted globe, it must have 
passed through the same phases as the moon, with this very 
important difference, that the whole surface during its progress 
was subject to the denuding action of waters and from the first 
had valleys and sedimentary rocks in progress. 2. Certain con- 
clusions regarding the “origin of the mineral constitution of 
igneous rocks,” including the idea that differences in fusibility will 
determine the mineral combinations, and that ‘“‘the same igneous 
rocks may occur of all ages, etc.’’ 3. As to origin of continents 
“the areas of the surface constituting the continents were first 
free from eruptive fires. These portions cooled first, and conse- 
quently the contraction in progress affected most the other parts. 
The great depressions occupied by the oceans thus began ; and for 
a long period afterward, continued deepening by slow, though it 
may have been unequal progress.” He cites the evidence of 
elevations all along the geological history, the presence of marine 
fossils in elevated upturned strata and the depression of the oceans 
in the coral islands, quoting the observations of Mr. Darwin, and 
referring to his own, to be reported on later, in confirmation of 
these views. The outline of the system is tersely expressed in 
the closing sentence of this article: 
“The principles explained place the general theory of change of 
