94 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
shown in the illustration just referred to. All the forms mentioned 
above, from simple corrugations or ropy forms, as they are frequently 
termed, up to hollow folds 20 or more inches high, occur in abundance, 
especially near the sources of the lava streams. It needs no argument 
to convince the visitor to the Cinder Buttes that various forms were 
produced by the subsurface flow of the lava, which caused its highly 
viscous surface to become corrugated and folded. The most interest- 
ing fact is the tearing away of the nearly rigid crust from the more 
viscous portion below and its upward bending into hollow folds. In 
all these instances the direction of the force acting on the stiffened 
but still yielding crust was horizontal and slowly applied. Variations 
in the results dependent on the degree of rigidity of the crust, its 
thickness, the rate of motion, etc., are numerous. 
In all of the examples thus far referred to the crust was only 1 or 2, 
or perhaps 3 inches thick and still plastic and yielding to a slowly 
acting force. Instances are also numerous where the crust was 2 or 3 
feet thick, but still not brittle, and was pressed up into folds that are 
6 to 10 feet high and perhaps 15 or 20 feet across at the base. Ridges 
of this nature form a characteristic feature of the topography of the 
lava surfaces, and frequently have caverns beneath them through which 
a person can walk with freedom. The sides and roofs of the caverns 
are exceedingly rough and frequently bear evidence that the lava was 
still sufficiently mobile to flow after they were formed. The roofs in 
many instances are fretted with stalactite-like pendents of congealed 
lava. The sharpness of the projections is sometimes blunted by the 
subsequent flow of the lava and its gathering into more or less dis- 
tinct drops. In one instance I found the floor of such a cavern to be 
corrugated for a distance of 20 feet or more, showing that there had 
been a flow of the lava along it. This occurrence suggests that corru- 
gation in the case of old lava flows is not a positive indication that such 
markings belong to the actual surface. - 
The ridges and domes just referred to sometimes have cracks along 
their summits parallel with their larger axes, which it will be remem- 
bered are transverse to the direction of the force that produced them. 
In some instances the sides of such a ridge slope sharply, like the roof 
of a house, and there is a break on each margin where the descending 
slope meets the adjacent flat or nearly flat surface. In these cases it 
is evident that the crust had become too rigid to yield to the tangential 
pressure applied to it without breaking, and fracture and displace- 
ment resulted. The lower surfaces of these inclined blocks present 
the characteristic "pulled dough" forms referred to in the case of the 
hollow folds, showing that owing to lateral pressure the rigid crust I 
had been forced or pulled away from the more viscous material 
beneath. 
Openings in lava similar to those just described are mentioned by 
