114 The American Geologist. 
August, 1905 
Grand canyon group), and the basal complex. The upper series 
consists of shales and limestones. Below this, with an erosion in- 
terval, is the second, consisting of sandstones, with interbedded and 
cutting basic eruptives. Inferior to this series, and separated by 
a great unconformity, is a set of thinly bedded and nearly vertical 
quartzites of undetermined thickness, broken by intrusive masses 
of granite. These three are clearly clastic series. The basal com- 
plex as described by Powell and Gilbert consists of thoroughly crys- 
talline hornblendic and micaceous schists, gneisses, and granites, 
like the fundamental complex of the remainder of New Mexico and 
Arizona. Between this basal complex and the Vishnu series, as 
shown by Powell, is a vast unconformity. We have then in this 
region passing from the base upward, a fundamental complex; 
great unconformity; quartzite series of unknown thickness (Vish- 
nu); great unconformity; Grand Canyon series; minor unconform- 
ity; Chuar series; great unconformity; Cambrian." 
There are then recognizable in the Grand Canyon part 
of the region at least fonr great unconformities in the space 
between the undoubted fundamental complex of Archaean 
age and the Cambrian sandstones. Each of these four un- 
conformities represents a long period of time when the rocks 
were elevated above the sea, flexed and then subjected to 
enormous denudation. Powellt has estimated that in the 
case of the latest of the intervals mentioned which is repre- 
sented at the base of the Cambrian sandstones, at least 
10,000 feet of beds were bowed up, contorted and eroded in 
such a manner as to leave but fragments in the synclinals. 
Each of the great unconformities represents similar con- 
ditions. In New Mexico these four periods of enormous 
erosion were probably superimposed. The elastics of the 
Proterozoic must have suffered tremendously. Over very 
large areas every vestige must have been removed. In all 
likelihood only scattered remnants remained. Thus in ad- 
joining mountain ranges the crystalline basement may be of 
Archaean age in the one case, while in the other it may be 
Proterozoic. 
The differentiation of the fundamental Azoic complex 
from the Proterozoic crystallines must rest upon the appli- 
cation of some such scheme of critical criteria as has been 
so successfully formulated in the Lake Superior region. 
While there is as yet much uncertainty regarding the 
, L T". S. Geog. and Geol. Sur. Terr, 1876. 
