eussell.] PAHOEHOE. 91 
(2) The other most prominent kind of lava stream is the aa. The aa streams 
have no upper flow-like surface; they are beds of broken-up lava, the breaking of 
which occurred during the flow. They consist of detached masses of irregular 
shapes, confusedly piled together to a height sometimes of 25 to 40 feet above the 
general surface. The size of the masses is from an inch in diameter to 10 feet and 
mere. The lava is compact, usually less vesiculated than the pahoehoe, not scoria- 
ceous; but exteriorly it is roughly cavernous, horridly jagged, with projections often 
a foot or more long that are bristled all over with points and angles. In some cases 
ragged spaces extend along planes through the large masses, like those of the exte- 
rior; but in these, as in other parts, it is evident that the agency was tearing and 
upploughing and cavity making in its action, and not vesiculating. 
Pahoehoe. — The lava surface having the characteristics included 
under the term pahoehoe occurs especially near the sources of the 
lava streams, and in some instances continue for several miles down 
their courses, and in certain cases are not entirely absent even at their 
distal extremities. The leading features are broad, generally smooth 
swells and stream-like ridges with convex surfaces, which cross each 
other as if braided together, but begin and end indefinitely. Some 
one has compared the appearance of such confluent outflows of lava to 
a mass of giant slugs, crawling over each other. The simile is cer- 
tainly suggestive. Slow motion of many, generally parallel, but occa- 
sionally diagonal and recrossing streams, from 2 or 3 to 10 or 15 feet 
in width, united into a single generally flat sheet, with perfect pres- 
ervation of form in their central parts, convey some idea of the gen- 
eral appearance of much of the pahoehoe about the Cinder Buttes. 
The origin of the peculiar and highly characteristic pahoehoe surface 
is due to the flow of viscous lava, which in consistency resembled 
asphaltum or pitch. The onward motion was not a continuous flow, 
as in the case of thoroughly liquid substance, but the surface and front 
of the advancing stream stiffened and bulged upward, and the more 
thoroughly molten material within broke through the tenacious but 
still plastic surface portion and advanced as a well-defined stream for 
perhaps a few yards or rods, and in turn stiffened at the surface and 
expanded on account of pressure from within and halted with a curved 
front bulging outward. These slow-moving, viscous streams crossed 
one another, but became more or less thoroughly intermingled. This 
manner of progression, accompanied by many variations in detail, 
can be plainly seen on the congealed surfaces now remaining, especially 
near the sources of the lava stream. 
While the curved and swelling forms so characteristic of pahoehoe 
surfaces are apparently smooth when seen from a distance of a few 
rods or a few yards, they are rough to the tread, and on close examina- 
tion are found to be braided over with low ridges and string-like forms 
inclosing or bordering shallow depressions, as is well shown on PI. 
XIII, A. The origin of this coarse lace work on the surface of lava 
which cooled slowly, "becoming highly vesicular before its sluggish 
