TU-SEIAR STRUCTURE VOLCANIC STRUCTURE. 19 



and flanks with irregular and overlapping patches of lava. Thus lava bed 

 is imbricated on lava bed, but unlike the tiles of a roof, the upper edge of 

 the lower sheet is placed on the lower edge of the upper. This struc- 

 ture is well represented in the Uinkaret Mountains in Northern Arizona, and 

 has been more fully discus/sed by me elsewhere, vide The Exploration of 

 the Colorado River, &c., page 1 99 et seq. 



IX. TU-SHAR STRUCTURE. 



When a plain or valley which receives extravisated material from below 

 remains at a base level of erosion during the period of successive eruptions, 

 flood of lava is piled on flood of lava until a vast mass of material is accu- 

 mulated 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 irregular mass, 

 due to its degree of fluidity arid 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. 



The volcanic activity being in a region where movements of displace- 

 ment are in progress, it is often the case that the structure of this class- of 

 mountains is greatly modified by such displacements. Mountains composed 

 of such irregular beds of lava are of frequent occurrence in the region 

 under discussion. A fine example is seen in the vicinity of the town of 

 Beaver, Utah Territory, in what are known as the Tu-shar Mountains. 



X. VOLCANIC STRUCTURE. 



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 in the region under discussion. Great 

 complex cones such as are found in other parts of the world do not occur, 

 but a few double and one triple cone has been observed. The great majority 

 of the cones observed are built of cinders on broad sheets of lava, and are in 

 fact concomitant forms of lava mesas. Such cones are comparatively ephem- 

 eral, as the scoria and ashes of which they are composed yield readily to atmos- 



