18 
THE GEAJTD GASON DISTRICT. 
far as the Aubrey Cliffs, 150 miles to the southward, and has a width of 
100 miles. 
The region occupied by the terraces has a width from north to south of 
about 30 to 40 miles, and a length from west to east of nearly 100 miles. 
The trend of the cliffs is in detail very iiTegular, making long detours and 
throwing out great southward projections. Still there is much of system 
in their arrangement, and a rude kind of parallelism in the trends of their 
escarpments. In a general way their course is from west to east, reaching 
from the Hurricane Ledge to the valley of the Paria. It is desirable to follow 
the course of these cliffs beyond the Paria and note the manner in which 
they terminate the strata along the eastern side of the district. 
At the Paria River the limiting edges of the Mesozoic strata suddenly 
change their trend and strike off to the southward, or rather in courses a 
little east of south. For more than 140 miles they form the eastern bound- 
ary of the Grand and Marble Canon area. Their topographical features, 
however, are very different from those presented upon the southern flanks 
of the High Plateaus. Generally each terrain (Trias, Jura, Cretaceous, &c.,) 
ends in cliffs facing the west and overlooking the great Carboniferous 
platform through which the Grand and Marble Canons are cut; but the 
regular terraces, rising step above step, are wanting. The arrangement can 
be understood only by consulting the geological map and sections. It is 
greatly complicated by a large monoclinal flexure (Echo Cliff flexure) which 
throws down the country east of the Marble Canon and Paria Plateau, and 
upon the slope of this monocline appear the edges of the Trias and Jura, 
and a little beyond it the Cretaceous. We are here concerned, however, 
only with certain facts which are common to the regions north of the dis- 
trict and to those east of it. On both of these sides the edges of the 
Mesozoic strata, ending in long lines of clift' above cliff, face towards the 
great Carboniferous platform. 
And while this conception is fresh, let us tiu-n to the western and 
southern sides of the district, where the Carboniferous platform itself ends 
in giant cliffs which overlook the sierra country beyond. In that sierra 
region the rocks are all of older age — Archaean, Silurian, and Devonian (f), 
and the elevations, except the summits of the ridges, are much less than 
