446 G. F. LOUGHLIN 
with a very gentle and uniform easterly dip, as if it had slid bodily 
westward over the quartzite and shale; but, as the writer entered 
the canyon at sunset he had no opportunity to get more than a 
hasty glance at the structure. Mr. Emmons remarked that the 
structure, as seen from the canyon bottom, appeared to be a strong 
unconformity, but that examination the whole length of the canyon 
proved the structure to be an §-shaped fold with only its upper 
half exposed.t The limestone cliffs show the same succession of 
limestones as that which overlies the Cambrian quartzite and shale 
in the Cottonwood district, and the quartzite exposures at Provo 
are therefore to be assigned to the Cambrian, and not to the Devo- 
nian as was done by the Fortieth Parallel Survey. 
The southern quartzite occurrence, southeast of Provo, also 
forms an arch along the range front, cut by Slate Creek Canyon. 
In this case, however, the western limb is very steep. The quartz- 
ite is exposed approximately along the axis and is flanked by over- 
lying limestone dipping steeply westward. The east limb, which 
can be studied along the walls of Slate Creek Canyon, dips very 
gently and is overlain by about 200 ft. of shale and the regular 
succession of limestone beds. The conglomerate bed with light- 
gray limestone and other pebbles is exposed in the canyon near its 
mouth, and beneath it in the creek bed is a doubtful outcrop of 
dark bluish-black limestone similar to that flanking northern quartz- 
ite exposure. The evidence in the two cases, though not abso- 
lutely convincing, is thus consistent and points to the existence of a 
local limestone horizon below, or within, the quartzite, and also 
to the existence of an unconformity below the exposed part of the 
Provo section, presumably in Lower Cambrian time.? Limestone 
beds of Middle Cambrian, and one small bed of Lower Cambrian 
or even earlier age, were found by the writer intercalated in the 
great quartzite series of the Simpson Mountains north of the 
1 Op. cit., pp. 345-46. 
2 According to Walcott (‘‘Cambrian Sections of the Cordilleran Area,” Smithsonian 
Inst. Misc. Coll., 1908), the first 100 ft. of shale above the basal quartzite in Big Cot- 
tonwood Canyon are Lower Cambrian and next 150 ft. are Middle Cambrian. The 
quartzite at Provo, 25 to 30 miles farther south, may therefore also be regarded as 
approximately at the top of the Lower Cambrian. 
