russell.J CORRUGATIONS AND ARCHES. 93 
the lava was nearly solid at the time the motion producing the folds 
took place. 
Not only simple but compound ridges were frequently formed in 
the manner just stated. In the case of the compound ridges it is 
Apparent that a congealing of the surface, accompanied by the forma- 
tion of a series of small wrinkles, was followed by the stiffening of a 
thicker layer as cooling progressed. This thicker sheet in turn be- 
came wrinkled, the larger folds carrying on their surface the smaller 
wrinkles, similar to the occurrence of small waves on the surface of 
a larger one. In these instances, also, the larger folds are frequently 
inclined forward. Still other complexities occur, as when after a 
series of simple or compound wrinkles had been formed the direction 
of the underflow changed and a new series of wrinkles and folds was 
produced which cross the earlier series diagonally. 
The corrugated like the adjacent un wrinkled portions of the pahoe- 
hoe surfaces are blistered, as explained above, by the breaking of the 
thin surfaces of steam cavities, and cord-like ridges produced by the 
elongation of the material between the shallow depressions. In such 
instances the ridges between the blisters were still more elongated and 
frequently pass from side to side over the curved crest of a fold, pro- 
ducing elevations that suggest the appearance of the thread on a screw. 
The ridges are parallel, however, and not spiral. 
Selecting other examples to illustrate the series, we find that many 
folds rise sharply from the general surface to a height of 10 to 20 or 
more inches. In these instances the sides of an individual fold are 
compressed, while the top may be expanded, so as to reveal in cross 
section much the appearance of the end of a railroad rail, as is shown 
in PI. XIV. At times each member of a series of ridges of this nature 
is inclined forward in the direction of How. The expansion of the 
summit portion of a fold of the nature just referred to is the first in 
a series of changes that produced surprising results. The next varia- 
tion is where the stiffened crown of a fold has separated from the 
more plastic material in its inner portion and been torn away from it, 
leaving a hollow space, as is shown on PI. XIV. The expanded sum- 
mits of such ridges are long, hollow tubes. A still greater movement 
resulted in the formation of larger folds, in w T hich the hollows are 6 to 
10 inches or more in height and 3 to perhaps 5 inches across, as is 
shown on PI. XIV, B. The inner surfaces of these hollow folds pre- 
sent the appearance produced when a high viscous substance is pulled 
asunder, as, for example, when a loaf of bread not completely baked is 
broken apart. In these instances the inner walls of the cavities are 
exceedingly rough, presenting numerous sharp ridges and projecting 
points, and are frequently crossed b} r rods and filaments of brittle 
lava, which are small in the center where most elongated and expand 
where they merge with the walls. This appearance is imperfectly 
