256 HAWATIAN ISLANDS. 
We may easily trace out the steps in the formation of the rock 
represented in this last figure. a’ was once near the limit of the bank. 
It was swept over by a gale or a wave from the sea, and reduced to the 
outline aa’. A series of driftings parallel with the surface followed, 
during more quiet weather, raising it to the line 6 5’; and afterwards, 
by a lighter breeze still, the sands were wafted but part way over, 
producing the shorter layers c. ‘Then, by an increase of the winds, 
the whole surface was again covered, increasing the hillock both in 
breadth and height. Thus we might pursue it through its recorded 
history. Some minor irregularities may have been caused by eddies 
around twigs growing in the sand. 
The same shores that now present us with these coral sand-rock 
ridges or hills, present also recent sand-hills near the beaches. At 
Laiée these recent drift-heaps are twenty-five feet high, and extend 
back over a surface one hundred yards in width. They have an un- 
even curving surface above, which is continually varying with the 
force or direction of the wind. ‘They do not form a continuous ridge 
parallel with the shore, but a series of hills, with valleys between, 
trending about east-by-south and west-by-north, while the coast-line 
faces northeast. One of the long sand-drifts had been cut through 
by the winds, and the gully formed had a vertical front on the south 
side ; it was again slowly filling up. 
This irregular subdivision into hills, elucidates the features of 
the ridges of sand-rock. Before studying this example offered near 
by, the occurrence of so many isolated hills instead of a continuous 
ridge, seemed to be the greatest difficulty in the way of accounting 
for their formation. We farther learn from the recent drifts, that 
shells are of very rare occurrence in them. Grass is growing thinly 
over them, and the surface is often scratched in interrupted lines by 
the fleet ocy pod. 
The character of the lamination illustrates the formation of siliceous 
sand-rocks, and all rocks that have proceeded from material drifted 
by winds. ‘The frequent changes in the direction of the lamination 
well exemplifies the structure of the Sydney sandstone of New South 
Wales. 
5. GENERAL REMARKS UPON THE GEOLOGICAL HISTORY OF OAHU. 
There are several facts in the structure of Oahu pointing towards 
one and the same theory of its origin, urging us to conclusions which, 
at the first glance, may appear incredible. 
