406 BOTANICAL GAZETTE [NOVEMBER 
A-a is exceedingly rough lava (fig. 18) composed of incoherent, 
bristling spiny blocks of all sizes, like gigantic clinkers. DANA 
says (loc. cit., p. 162): 
They look as if the mountain had been shattered to a chaos of ruins. The 
fragments vary from 1 to 10,000 cu. ft. or from a half bushel measure to a 
house of moderate size. They are of all shapes, often in angular blocks, some- 
times in slabs, and have a horrible roughness beyond conception, points and 
angles standing out in every direction; they lie together, touching only by 
their edges or points, leaving deep recesses everywhere between them. 
Fic. 17.—Front or end of pa-hoe-hoe flow in crater of Kilauea; note smooth 
character of surface and lobular margin; plants here aa there along margin are 
Vaccinium reticulatum. 
The beds are 30-60 ft. thick, with innumerable cavities between 
the blocks. On very old a-a flows the blocks and finer material 
have settled somewhat, rendering the entire mass more compact; 
the surface material is also less spiny. A-a flows are so rough as 
to be practically impassable by man or beast, and in traveling 
across the lava wastes one is compelled to make long detours to 
avoid them. The sharp cutting edges quickly destroy even the 
