THE 
GEOLOGICAL MAGAZINE. 
NEWBIES. (DECADE Tha VOEw xX. 
No. VIL—JUNE, 1883. 
(jz aS EIN, Va Ne Cees 
ad ee 
I.—OricIn or ConrTiNENTSs. 
By W. O. Crospy, Esq. ; 
Of the Museum of he Boston Society of Natural History, etc., etc., Boston, U.S.A. 
HE theory of the origin of continents and ocean-basins deve- 
loped during the last third of a century, chiefly by Prof. Dana, 
and commonly known as Prof. Dana’s theory, is now accepted by 
many geologists. The main points in this theory, as gathered from 
the latest expression of Prof. Dana’s views, are the following :'— 
The earth, superficially at least, is, and was originally, before it had 
a solid crust, of unlike composition on different sides. This hetero- 
geneity caused a corresponding difference in heat-conductivity. The 
more rapidly conducting areas cooled fastest and were the first to 
become covered with a solid crust. Solidification is attended by 
contraction ; and therefore the newly formed crust must have been 
heavier than the liquid immediately beneath it. As a consequence, 
it broke up and sank until it reached a liquid stratum of the same 
specific gravity as itself; and afterwards the process of crusting and 
sinking went on until a solid crust was built up from this point to 
the surface. Through the continued escape of heat this primitive 
crust is thickened, and is still thickening by additions to its lower 
surface. ‘These first formed portions of the crust became, and will 
always continue to be, the continents. The remainder of the earth’s 
surface was still liquid, after the solidification of the continental 
areas was well advanced; and, of course, as long as it continued 
liquid, its surface was level with that of the crust-areas. Finally, 
it became the theatre of a similar process of crusting and sinking, 
and at last permanently froze over. Now the main point is that the 
contraction of this inter-continental crust during its formation caused 
its surface to sink below that of the continents; and the depressions 
thus developed became the future ocean-basins, which, like the 
continents, are necessarily of a permanent character. Indeed, it is 
a plain deduction from Prof. Dana’s theory that the existing con- 
tinents and oceans are as old as the earth’s crust; and that during 
the course of geological time the continents have become constantly 
wider and the oceans deeper. 
This hypothesis has to a considerable extent supplanted that 
held by Lyell and the earlier geologists, according to which the 
continents and ocean-basins are great upward and downward 
bendings of the earth’s crust, and not necessarily fixed. Many of 
the ablest geologists, however, still hold that the old theory has 
1 Amer. Journ. Se. (3), vols. v. and vi. 
DECADE II.—YOL. X.—NO. VI. 16 
