426 J. W. Powell—Types of Orographic Structure. 
ing patches of lava. Thus lava bed is imbricated on lava bed, 
ut unlike the tiles of a roof, the upper edge of the lower sheet 
is placed on the lower edge of the upper. This structure is 
well represented in the Uinkaret Mountains in Northern Ari- 
zona, and has been more fully discussed by me elsewhere. (Vide 
The Exploration of the Colorado River, &c., page 199 e¢ seq.) 
1X.— Tu-Shar Structure. 
When a plain or valley which receives extravasated material 
from below remains at a base level of erosion during the peri 
of successive eruptions, flood of lava is piled on flood of lava 
until a vast mass of material is accumulated from which the 
rains and streams carve mountains. The several beds of which 
such a mountain mass is composed are exceedingly irregular, 
from three causes: first, each bed as poured out was an irreg- 
ular mass, due to its degree of fluidity and the character of the 
ground on which it was poured; second, each bed was more 
or less modified by erosion, which occurred after it was poured 
out, and before it was covered by a subsequent flood; and, 
third, the general mass has been eroded to a greater or less ex- 
tent in producing the present forms. 
e voleanic activity being in a region where movements of 
displacement are in progress, it is often the case that the struc- 
When many eruptions come successively from the same vent, 
and each is a comparatively small amount, cones are built. 
Cones of such simple structure are of frequent occurrence 10 the 
as 
fully corroborate the conclusion. From such evidence we are 
ity in the 
rovinces. If the human history of America could 
