J. D. Dana on Denudation in the Pacific. 55 - 
vent is mended by the force which made it. It is, therefore, a 
gratuitous assumption that such fissures have been common. 
The existence, however, of large valleys such as have been attrib- 
uted above to convulsions cannot be doubted; but the sea wonld 
exert its power in such places, nearly as now in Fangaloa Bay, 
Tutuila, and other bays in continents ;—a beach forms, and a 
shore plain, and afterwards there is a little action from the sea 
in these confined areas of water. 
In the Illawarra district, New South Wales, there are several 
places where dikes of basalt have been removed by the sea, and 
channels one hundred yards in depth, of the width of the dike 
(six feet), now exist, cutting straight into the rocky land. This 
is an example of the action of the sea where everything is most 
favorable for it. And we observe that there is little resemblance 
in this narrow channel with but a trifling wear of the inclosing 
rocks, to the valleys which are to be accounted for in the Pacific ; 
and little authority to be derived from it for attributing much 
efficacy to the sea in wearing out valleys. The reason of this . 
is apparent in the fact that the sea rolls up a coast in great. 
swells, and cannot parcel itself off, and act like a set of gouges: 
this latter effect it leaves for the streams and streamlets of the 
shores which are gouges of all dimensions. 
Although the sea can accomplish little along coasts towards 
excavating valleys, yet when the land is wholly submerged, or 
only the mountain summits peer out as islands, the great oceanic 
currents sweeping over the surface and through channels between 
the islands, would wear away the rocks or earth beneath. From 
the breadth and character of such marine sweepings, we learn 
that the excavations formed would be very broad rounded valleys ; 
aud their courses would correspond in some degree with the prob- 
able direction which the currents of the ocean would have, over 
the region in case of a submergence. Moreover where there are 
different open channels for the ingress of the sea, having free 
intercommunication, there are often strong currents connected 
with the tides, and consequently much erosion. It is obvious 
that the valleys of the Pacific islands have nothing in their fea- 
tures or positions attributable to such a cause. ps 
Running water of the land, and gradual decomposition.—Of 
the causes of valleys mentioned in the outset we are forced to rely 
for explanations principally upon runping streams: and they are 
not only gouges of all dimensions, but of great power, and in 
constant action. ‘There are several classes of facts which support 
Us in this conclusion. : 
a. We observe that Mount Loa, whose sides are still flooded 
With lavas at intervals, has but one or two streamlets over all its 
slopes, and the surface has none of the deep valleys common 
about other summits. Here volcanic action has had a smoothing 
