230. BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
flows across the old fault, on one side of which is hard Aubrey limestone 
while on the other is soft Moencopie shale. In other words, when the 
lava was poured out the country was so far worn down toward base- 
level that the difference in the two kinds of strata was of comparatively 
little importance, At present, where these two formations lie side by 
side above base-level and are exposed to erosion, the shale is very rapidly 
worn away, leaving the limestone as an escarpment. At the time of the 
lava flows the junction of the two seems to have been so close to base- 
level that the hard and soft strata on the two sides of the fault were 
worn down to a level or only gently sloping surface, and lava from the 
downthrown side flowed across the fault line. This does not mean that 
the entire region was reduced to the lowest possible level, for near the 
Virgin river the lava after crossing the fault was soon checked by an 
escarpment of limestone rising two or three hundred feet with a fairly 
strong slope. 
At Sugar Loaf, the third of the localities already mentioned, and on 
the Shivwits plateau there is evidence of a local base-levelling of a more 
pronounced type. Here the surface at the time of the lava flows previ- 
ous to the recent faulting consisted of Moencopie shales. These are 
very soft rocks which, under present conditions of high altitude and con- 
sequently of active erosion, never form a level surface of any considerable 
extent. As soon as the protecting cap of Shinarump sandstone and 
conglomerate is removed, the shales are dissected into a regular bad-land 
topography and erelong melt away entirely. Yet under the lava flows 
there is an extensive flat surface of just such shales, which does not 
even correspond with the bedding planes of the rock, but bevels the 
strata at a slight angle, and so cannot be due to any particular layer of 
unusual hardness. Such a surface can only be produced close to base- 
level. As it is best exemplified in the Shivwits plateau which lies in 
the centre of Mohave county, Arizona, we shall hereafter refer to it as 
the Mohave peneplain. 
Farther north between Toquerville, and Dry canyon, a lava flow some 
fifteen miles long and from two to three miles wide lies partly in the 
valley at the western base of the Bellevue ridge and partly high up on 
the top of the ridge. The two portions have been broken apart and dis- 
placed from one thousand to fifteen hundred feet by the recent Hurri- 
cane fault. The lava seems to have come from a group of craters near 
the northern end of the flow, and located in part on the upheaved and in 
part on the dowuthrown side of the fault. The basalt from these flowed 
southward down the Bellevue valley. On the west it was checked by 
