138 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
wrote nearly thirty years ago, one cannot “imagine an erosion which 
would leave an abrupt wall of 7,500 feet in height on one side of a 
valley nearly twenty miles wide” (345). Hence wherever these theoreti- 
cal consequences are borne out by facts of direct observation, block fault- 
ing is thereby given so high a degree of probability while other processes 
are rendered so highly improbable that the theory of block faulting may 
be looked upon as well introduced, at least. 
The best examples that came under my observation of actual forms 
which match these preliminary members of the whole series of deduced 
type forms were not among the Basin ranges proper, but along the bor- 
dering Wahsatch mountain front, by which the Great Basin is limited on 
the east. The mountain base near Provo and near Ogden deserves care- 
ful study in this respect. 
The Wahsatch range is divided into several local mountain groups by 
the canyons of streams that rise a number of miles east of the line of 
higher summits and flow westward to Salt Lake Basin. In the neigh- 
borhood of Provo, the canyons are those of Spanish fork, Hobble creek, 
and Provo river, between which the mountain groups may be called the 
Spanish peaks, Wahsatch, and the Provo peaks Wahsatch, or more briefly 
the Spanish and the Provo Wahsatch (Emmons, 340, 344). 
Close by Provo, where my party had the most leisure for attention to 
this problem and where we had the advantage of guidance by Prof. E. 
H. Hinckley of the Academy in that city, the expectations of theory are 
extraordinarily well supported by the facts. The mountains spring 
boldly from the plain ; their base line breaks obliquely across the tilted 
and folded rocks of the mountain mass: the occurrence of a base-line 
fault is explicitly stated by Emmons (345). Views of the Provo Wah- 
satch from the roof of the Academy in Provo looking northeast and 
southeast are given in Plate 4. We made an excursion up Rock can- 
yon, Plate 4, a, to a mid-monoclinal ridge back of the frontal summits, 
and returned by Slate canyon, Plate 4,8. There are some indications 
of faults in the longitudinal valleys between the monoclinal ridges 
(Emmons, 345, 348), but nothing at present known serves to give 
date to these faults, should they be proved to occur. Figure 4 shows 
the generalized structure thus determined: an anticlinal axis lies 
near the western base of the mountains opposite Provo, while a 
great monocline, the eastern half of the incomplete anticline, consti- 
tutes the rest of the range. Further south, the anticline is not seen 
at the mountain base. The rocks in the anticlinal axis are said to be 
mid-Palzozoic ; those of the crests are Carboniferous (Emmons, 345, 
