154 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
indicated by the maturity of the valleys which dissect the uplifted rocks 
east of the fault line. These side valleys are as thoroughly mature as 
is the main valley, which is to be expected if the fault is old, and if 
both main and branch valleys developed simultaneously after a period 
of baselevelling ; but which is incompatible with the idea that the main 
valley is ancient, the fault recent, and the branch valleys still more 
recent. A view of one of these side valleys is shown in plate 21, 
figure 2. It may be contrasted with the young valley shown in 
plate 20, a valley eroded in the uplifted block along a part of the 
Hurricane escaqmient where the faulting is truly recent. 
Hurricane Fault. 
Our observations along the Hurricane Fault were for the most part 
confined to that portion of its course north of the Grand Canon district, 
between the High Plateaus and the Great Basin. Such facts as we 
noted in the Grand Caiion district related to features already described 
by Davis ('01, p. 146-147; '03, p. 26-31), and by Huntington and 
Goldthwait ('04), but are in part presented below because they led 
us to an interpretation which differs in certain details from the con- 
clusions reached by Huntington and Goldthwait. 
It has been shown by the authors above mentioned that the "Hurri- 
cane Ledge" is in the main a fault-line cliff, developed by erosion of 
softer beds west of the fault after baselevelling had destroyed the 
original topographic effect of faulting; but that at a more recent period 
movement has taken place along part of the fault line, so that the basal 
portion of the cliffs in the Toquerville district is of true fault cliffs. In 
other words, a part, of tlie Hurricane scarp is believed to have been 
exposed by erosion after the occurrence of an ancient fault movement; 
while part is believed to have been exj^osed recently as the direct result 
of a recent fault movement along the same ]:)lane. 
We recognize that the two fault movements above indicated have 
occurred, but believe that the features in the vicinity of the Virgin 
River, as described by Huntington and Goldthwait, and as observed 
by us in the field, indicate a third period of faulting quite as distinct 
as the two others. 
A few miles south of the Virgin River, a lava-capped butte of Per- 
mian shales is found on the verv edge of the limestone cliff's of the Hurri- 
