246 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
recent. The oldest lava sheet, A, appears to have been poured out 
on the graded surface which characterized the country at the end of the 
inter-fault cycle. Then the general uplift which seems to have accom- 
panied or perhaps preceded the recent faulting and to have raised the 
whole region—the western as well as the eastern side of the fault. 
Erosion at once became active, and valleys were cut to a depth of three 
hundred and fifty feet on one or both sides of the lava sheet. Then, 
before the uplift had been completed, or at least before the renewed 
erosion which it occasioned had reached a condition of equilibrium, 
another stream of lava, B, flowed down the valley east of the first. 
Again valleys were worn away, this time to a depth of nearly three 
hundred feet, and in very recent times a third lava flow, C, poured down 
A 
Figure 11. 
Lava terraces at St. George. 
the western valley. Since that time there has been further erosion 
along the main drainage channels, so that the first lava flow now stands 
some eight hundred feet above the neighboring streams. Elsewhere are 
even more modern lava flows, showing absolutely no sign of erosion, 
so that we have good evidence that there has been a continuous series 
of voleanic eruptions extending throughout the whole period that may 
be geologically called recent. 
The difference in elevation at St. George between the Virgin river and 
the most elevated lava sheet gives a minimum measure of the amount 
of uplift to which the downthrown block west of the Hurricane has been 
subjected. This amounts to eight hundred feet. As the river can 
hardly have flowed more than one or two hundred feet below the general 
surface at the end of the inter-fault cycle, the country must then have 
