98 SNAKE RIVEK PLAINS OF IDAHO. [bull. 199. 
pack on arctic seas. The congealing of a lava stream when covered 
with floating blocks of its own disruptured crust is similar to the 
refreezing of a river on which there are ice flows and ice jams. The 
submerged portions of the lava blocks are thus united by the harden- 
ing of the liquid or plastic lava in which they are immersed, and a 
breccia is formed. This second crust, however, is sometimes ruptured 
and the fragments of breccia are displaced and tossed about. 
The fact that in the production of an aa surface a hard crust is first 
formed, which is ruptured owing to the flow of still liquid lava beneath 
it, is again indicated by striations or grooves on the under surface of 
the crust fragments. These markings are frequently several feet long, 
have a width of several inches, and are usually straight. At first 
glance they resemble glacial scorings, but there is no polishing. The 
origin of these grooves can be traced to hard fragments carried by the 
underflow and pressing against the still plastic lower surface of the float- 
ing crust. At times a ragged kernel or fragment of rock is found at 
the end of a groove, showing that at last its motion was arrested and it 
stuck fast. 
As already suggested, whether a pahoehoe or an aa surface will be 
produced on a lava stream is determined by the ratio between rate 
of cooling and the rate of motion. But this ratio is not the same for 
different lavas. When a lava sheet cools without motion, neither a 
characteristic pahoehoe nor an aa surface is produced. Many of the 
older sheets of Snake River lava illustrate this; they are simply plane 
surfaces, composed of either vesicular or compact granular basalt. 
The explanation of the origin of aa adopted above was not accepted 
by Dana, a who suggests that the breaking of a lava crust may be due 
to moisture derived from the rocks over which lava flows and leading 
to quicker cooling in certain areas than in others. Such an occur- 
rence, however, even if proved to exert an influence, seemingly intro- 
duces a variation into a more general process, without supplanting 
the controlling conditions. 
LAY A CAVES. 
In the production of an aa surface, as already stated, a rigid 
crust formed on the surface of a lava stream, becomes broken by the 
flow of the still liquid lava beneath. If the crust is of sufficient 
strength, however, not to be broken by the underflow, the lava beneath 
may flow out so as to leave a cavern. As is well known, this has 
occurred in many instances. Examples of this class of caverns occur 
in the lava that came from Black Butte, near its source, where it had i 
formed by its own upbuilding a considerable gradient, but none were 
observed intact in the nearly flat lava fields about the Cinder Buttes, 
".l. 1). Dana, Characteristics of Volcanoes. New York, 1890, pp. 242-244. 
