92 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
motion finally ceased, is easily explained. Where the lava cooled 
without motion, the vesicles once occupied by streams are spherical or 
nearly so. Where the lava cooled and stiffened while still slowly flow- 
ing, the vesicles are elongated. In such instances one may frequently 
find cavities which were drawn out until the wall at the top became 
exceedingly thin and was finally ruptured, the material that formed 
it spreading and leaving an elliptical blister-like depression. Much of 
the lava with extended cavities has the appearance of molasses candy 
that has been pulled until it became fibrous. When motion continued 
after many of the vesicles at the surface of the lava had become 
ruptured, the material forming the elevated boundaries of the numer- 
ous blisters was extended into long, sinuous ridges, and the surfaces 
assumed the appearance shown in the photograph referred to. This 
process took place not only at the surface, but at intervals within 
the mass, as the lower part continued to flow after the surface had 
stiffened and ceased to move; or what seems to have been frequently 
the case, successive layers, each a few inches thick, acquired different 
rates of motion, increasing with depth, owing to increase in plasticity, 
so that the surface portion of a sheet became more or less completely 
fibrous throughout a thickness of several feet. 
Corrugations and arches. — The wrinkling of the surface of a lava 
stream after it became stiff and viscous, owing to the flow of the more 
plastic material beneath in the manner well known in the case of many 
recent lava streams, is abundantly illustrated about the Cinder Buttes, 
where several novel variations of the process were observed. 
The simplest corrugations occur on nearly flat or bulging pahoehoe 
surfaces, and appear as a series of concentric ridges an inch or two 
high, with a flat space perhaps 3 inches wide between them. The 
corrugations number possibly 10 and possibly 50 individuals. They 
are drawn out into the form of half an ellipse, or appear as a con- 
centric series of parabolic ridges, which are highest and best defined 
at the front, where the curvature is sharpest, and which fade at the rear, 
where the figure is open. These series of ridges are sometimes 10 or 
20 feet in length, are sometimes gently curved, but more frequently 
are long drawn out and reveal the course of a narrow, sluggish stream. 
Where the motion was greater the spaces between the ridges became 
narrower and curved downward, forming small troughs or synclines. 
Frequently the ridges adjoin one another, and are even tightly com- 
pressed and inclined in the direction of flow of the plastic material 
beneath, as is illustrated by the photograph reproduced in PI. XIII, B 
and O. That a flowing motion occurred in nearly solid lava which was 
yet capable of yielding to a strong force slowly applied is shown by the 
fact that in some instances the surfaces of the various layers distinguish- 1 
able in the folds are smooth and polished, having, in fact, the color and 
appearance of highly burnished blue steel. This polish implies that 
