496 MacCauGHEY: THE STRAND FLORA 
width, and the plants grow on top of the crevice, merely rooting 
in it. Others are several inches wide, and the smaller plants, such 
as Lipochaeta succulenta, Lepidium owaihense, and Cressa cretica, 
grow down within the crevice, only the upper parts of the branches 
showing above the rock. In the very large fissures,—one to three 
feet wide, the entire plant body may be concealed within the 
fissure. 
4. Lava boulder and pebble beaches——Wherever the shore line 
lava-sheets are subjected to the action of the sea, they are gradu- 
ally broken into massive boulders, which in turn are slowly ground 
into pebbles. These metamorphoses are abundantly illustrated 
along the windward coasts of Hawaii, Maui, and Molokai, and 
in such places as Kaena, Oahu, and Kilauea, Kauai. 
The boulders are usually 2-3 ft. in diameter, more or less 
oblate, smooth, black, very hard, heavy, and resonant. A beach 
composed of these ponderous rocks is very impressive, particularly 
during storm time, when the sea mills them with irresistible power. 
In various places, especially on the coasts of Kauai and Oahu, the 
lava boulders are consolidated in a calcareous matrix, formed of 
re-deposited coral lime. 
The seaward portion of a boulder beach is barren of terrestrial 
vegetation, as is to be expected, but the upper or landward por- 
tion, which is not disturbed by ordinary wave action, is the habitat 
of such forms as Sesuvium Portulacastrum, Ipomaea glaberrima, 
I. insularis, Euphorbia cordata, Tetramolopium sp., Kadua lit- 
toralis, and Wikstroemia Uva-ursi. 
The pebble beaches are relatively uncommon on Kauai and 
Oahu, but are more common on Maui and Hawaii. The pebbles, 
at the upper margin of the beach, are intermingled with soil, and 
the line of demarcation between beach and lowland is not distinct. 
5. Tufa beaches—Tufa craters occur here and there through- 
out the islands, from sea-level up to the highest mountain summits. 
In a few instances tufa cones stand so close to the shore line that 
the sea has cut beach platforms in their slopes. In these cases the 
strand is made of the solid wave-cut tufa rock. Leahi, Koko 
Head, Koko Crater, Manana, and Mokapu, illustrate this condi- 
tion. The tufa is soft and easily sculptured by the waves; it 
usually does not form boulders or pebbles, but fractures easily and 
