TOPOGRAPHY. 13 
along in a direct line to the northwest. The Marquesas also are 
mostly in a single range. The Tahitian Group, the Samoan, the 
Tongan, New Hebrides, New Caledonia, Salomon Islands, Radack 
and Ralick Islands, the Kingsmills, and the Ladrones are all distinctly 
linear groups. As this subject is one of both geographical and 
geological interest, and has been but imperfectly discussed in previous 
works, we pass in review the principal facts respecting the several 
groups of islands: and it will appear that its importance is not limited 
even by the Pacific Ocean, although nearly one-third of the whole 
surface of the globe; for it has an evident connexion with a system 
that pervades the world. 
The facts and any irregularities will be more correctly appreciated 
if the reader will first consider, with regard to ranges of mountains, 
that their courses often vary many degrees, even when a general 
linear direction is distinct. An exactly straight line is nowhere to be 
found, not even in a single ridge of a chain. This is apparent in any 
good map of the world. The peaks advance and retreat all along the 
line, and occasionally the mountains sweep around into some new 
direction, and then return again, more or less nearly, to their former 
course. Again we observe that there are often parallel ranges in the 
same chain, as is strikingly seen in the Alps, the Andes, and our 
own Alleghanies. 
The characters of fissures, or dikes, afford other hints that should 
be considered ; for they illustrate the operation of those internal forces, 
by which mountains have been uplifted; and even exemplify, as is 
generally admitted, the actual origin of many ranges of mountains. 
Facts illustrating this subject will be found in our descriptions of the 
Pacific Islands, and the Report on Hastern Australia. They show 
that fissures are generally a series of linear rents in some main direc- 
tion, and while they are often parallel, or in continued series, they 
are also sometimes arranged in a series of overlapping lines, and may 
be curved or straight in the separate rents, as well as curved or straight 
in the long composite ranges of rents.* They are often accompanied 
by transverse rents, at night angles with the general system. 
The several groups of islands may be considered in succession. 
a. Hawanan Islands.—The Hawaiian Islands -proper, extend from 
Hawaii to Nihau, (see preceding map,) a distance of four hundred 
* See American Journal of Science, ili. 2d Ser. p. 390. 
4 
