The Fundamental Complex — Keyes. 113 
in certain areas granites of later age, and these are more plentiful 
as the western part of Arizona is reached." 
In the light of the recent discoveries that in some of 
the mountain ranges of New Mexico portions at least of the 
crystalline foundation are of clastic origin, it becomes neces- 
sary to devise criteria by which the crystallines of the fun- 
damental complex (Archaean) may be separated from those 
that have a sedimentary origin. Until the application of 
these criteria to every mountain range is made, no general 
deductions can be drawn concerning the exact ages of the 
different crystalline formations. The reasons for this state- 
ment are obvious from even a casual examination of the 
basal crystallines in New Mexico. It is known that in all 
of the New Mexican mountains where the crystalline base- 
ment is open to view a marked unconformity exists at the 
base of the fossiliferous sequence. The late Carboniferous 
limestones generally rest directly upon the granites, gneisses 
and schists,, the foliation of which is more or less steeply in- 
clined or even vertical. 
That the erosion interval represented by the uncon- 
foimity was very long is quite evident. In southern New 
Mexico the early Carboniferous limestones begin to make 
their appearance. Then come Devonian beds, Silurian or 
Ordovician, and finally what appear to be Cambrian. A 
horizon of great unconformity persists under all the Pale- 
ozoics. 
Something of real significance of the old erosion plain 
becomes manifest by reference to the geological section dis- 
played in the Grand Canyon of northwestern Arizona to- 
wards the western border of the High Plateau region. The 
author* just quoted describes the following general condi- 
tions : 
"The Tonto sandstone of the Grand Canyon region, called by 
Powell and Gilbert Silurian in accordance with the nomenclature of 
the time, by present classification is to be placed as Upper Cam- 
brian. The great unconformity which separates this sandstone from 
the earlier series makes it very probable that the latter are pre- 
Cambrian. These inferior series in descending order are the Chuar, 
Grand Canyon. Vishnu series (together the equivalents of Powell's 
> hoc. cit.. p. 331. 
