ORIGIN OF THE GROUP. 281 
the Geology of the other Pacific Islands, and considering the conclu- 
sions to which we are thereby led. 
To comprehend fully the nature of the action alluded to above, we 
must keep in mind the facts already sufficiently illustrated, that fis- 
sures formed by subterranean forces are not long uninterrupted rents, 
but a serves of linear ruptures, approximately regular, separated by 
longer or shorter intervals, sometimes two or more being in parallel 
series, or one starting to the right or left of the point where another 
ceases; also, as is elsewhere remarked upon, that transverse fissures 
at right angles with the main line are a natural result of the same 
causes; also, the common fact, that fissures, after the first ejection, 
often remain open for a while, wherever widest or deepest, and con- 
tinue the ejections. These principles fully explain the double line 
of islands, which we have denominated the Loa and Kea ranges. 
They explain any irregularities in the lines; for perfect rectilinear 
courses would be highly improbable. They explain the existence 
of volcanic vents over the fissures, from which the several islands 
have been built up. ‘They explain the transverse trend of Niuhau 
and Kauai,* and of Kea and Loa. And to this we add, that the prin- 
ciple here referred to gives us a reason for the great prolongation 
of the shores of Hawaii to the south, in the line of Kea and Loa; for 
this region, like that beyond Kilauea to the east, appears to have 
been lengthened out, by lateral eruptions, beyond the proper limits 
of the dome Mount Loa. 
But this is not all. It is another common fact that a range of 
fissures has its maximum size at one of its extremities; and the 
subordinate rents of the series may have the same characteristic. If 
we may take as evidence of the extent of a fissure, the length of time 
which it subsequently pours forth lavas,—for plainly the deeper or 
wider it is, the more probability of its long continuing open,—we 
shall arrive at some interesting results respecting the Hawaiian 
Group. 
We observe in the first place that the southeastern extremity alone 
* This transverse direction is more nearly rectangular than it appears on the map ; 
the precipice of Nihau cuts off an eastern portion which, if present, would give this 
island a position more to the south of Kauai than it has on the map. As the two preci- 
pices, that of Niihau and that of Kauai are nearly in a single line, they afford another 
example in addition to that deduced from the relative positions of the two islands, of the 
rectangularity in the intersections between the transverse system of fissures, and the main 
line of the group. 
71 
