RUSSELL.] AA. 97 
from the flow of the subcrustal lava. Cases where the crust became 
fractured have also been cited. All this implies slow cooling and a 
gradual passage of the lava from a fluid through a viscid to a solid 
state, accompanied by a gentle flowing motion in the more slowly 
cooled subsurface portions of the lava streams. Should the crust con- 
geal rapidly so as to form a thin or comparative^ thin rigid sheet, 
while motion still continues, it is evident that it would become frac- 
tured and the blocks thus formed variously displaced; that is, an aa 
surface would be produced. 
Aa. — The presence on the Martin lava stream of angular blocks of 
basalt frequently united into a coarse breccia has already been men- 
tioned. In that instance an example is furnished of a lava stream 
having swift motion, owing to a high gradient. A similar occurrence 
throughout areas of many square miles, where the surface is generally 
level, may be seen over the lava streams which flowed away from the 
Cinder Buttes, and especially in the middle portions of their course. 
The areas here referred to have the characteristics included by Dana 
under the term aa. 
On the great lava flood which spread northward from near the 
northern base of the highest of the Cinder Buttes, but not from the 
volcano which built that eminence, the breaking of the surface begins 
within less than a mile of its source, and continues broadening to the 
northward as far as the eye can reach from the most commanding of 
the neighboring elevations. The surface of this stream is a chaos of 
angular blocks of scoriaceous lava, which at times are heaped in piles 
10 to 15 or more feet in height. In many instances large cakes of 
lava stand on edge, and are prominent landmarks from afar. The 
blocks of lava sometimes have wrinkled and corrugated surfaces, 
showing that they existed as pahoehoe before they were broken and 
displaced. In some places, in the midst of a broad area composed of 
fragments, there will be a small tract, usually depressed below the 
level of the general surface, which is smooth and unbroken and presents 
the features due to motion while } T et plastic. 
The general explanation of the origin of the aa surfaces, as is well 
known, is that a hard crust was formed on the lava while yet in motion, 
and that the friction of the still liquid portion beneath caused the crust 
to break. The fragments, being in most cases vesicular, were lighter 
than the liquid lava and were carried along by it. The analogy between 
such a lava stream and a river covered with cakes of ice is very close. 
In the case of a gently flowing lava stream, like a broad river covered 
with cakes of ice, the floated lava blocks are generally horizontal and 
only occasionally turned on edge. When the flow is stronger, as when 
the gradient down which the lava progresses is moderately steep, the 
lava cakes become heaped together and great confusion results. In 
such instances the piled-up lava blocks resemble an ice jam or an ice 
Bull. 199—02 7 
