44 GEOLOGY. 
This plateau is locally much broken and covered by floods of lava, which have flowed from 
the mountains we had passed; and yet, from commanding points of view, we could see that it 
had a distinct existence, stretching far away as a margin to the mountains, in a northwesterly 
direction. It is bounded on the east by a wall several hundred feet in height—in many places 
perpendicular, and generally abrupt—formed by the cut edge of a portion of the Middle Car- 
boniferous series. Having ascended this wall, we found our view again limited on the east by 
the bold escarpments of the edge of another and much higher mesa, which, with its salient 
angles, stretched away in magnificent perspective, both to the north and south. 
The surface of this mesa has an altitude of nearly 7,000 feet, due in part to a line of avhasiak 
which traverses it with a trend nearly N.NW. and S.SE. It is composed of the Upper Car- 
boniferous strata; the later members of that series forming the surface rock of a broad belt of 
country extending, from a point southeast of the San Francisco mountain, northwest across the 
Colorado into Utah. 
The great volcanic vent of the last-mentioned mountain has been opened up through this 
mesa, and has doubtless been an important agent in its elevation. Apparently little disruption 
has. been occasioned by it, but the floods of lava and heaps of ashes which have been thrown 
out of its many flues cover and conceal the underlying sedimentary strata in its vicinity. 
The surface rock of the high mesa dips rapidly toward the northeast, and forms the western 
slope of the broad valley of erosion through which the drainage of the northern declivities of 
the. Mogollon and San Francisco mountains was formerly carried to the Colorado; and of which 
the surface is now cut by the profound chasms, in the inaccessible depths of which flow Cascade 
river, and, through a part of its course, the Little Colorado. 
The opposite side of this valley is formed by a third mesa wall, which at the crossing of the 
Little Colorado is, with the slope at its base, at least 1,000 feet in height. This mesa is com- 
posed of deep-red sandstones, shales, and conglomerates, resting conformably on the Upper 
Carboniferous limestone, over which is a series of variegated marls, with bands of magnesian 
limestone. The latter series forms the surface of the mesa for many miles towards the north- 
east, and has an aggregate thickness of perhaps 1,500 feet. 
The variegated marls and the underlying red sandstones are all regarded as Triassic by Mr. 
Marcou; but the marls exhibit a remarkable lithological identity from top to bottom, and the 
upper portion contain plants of Jurassic affinities. Without more fossils from these formations 
it. seems to me, at least doubtful whether we can draw the lines of classification as sharply 
as he has ‘done; and it would even be a little surprising if there should ever be found good 
palwontological evidence for the identification of all the European subdivisions of the Permian, 
Triassic, Jurassic, and Chalk, of which he claims to have demonstrated the existence in this 
vicinity. 
Upon fhe t mesa of the variegated marls at the. Moquis villages rises still sesihens to the 
height of 800 or 900 feet, composed of coarse yellow sandstones, green shales, and beds of 
lignite—a group of strata which has been called Jurassic, but which contain impressions of 
dicotyledonous leaves, with Ammonites, Gryphea, and Inoceramus of Cretaceous species. 
These fossils leave no room for doubt in reference to the age of the strata which contain them, 
but. prove them to be Lower. Cretaceous. % 
‘This mesa is, geologically and physically, the highest cae we sein passed over on our 
route west of the Rocky mountains, Near Fort Defiance its summit has an altitude of nearly 
8,000 feet. It should also be said. that basin-shaped depressions on this mesa contain fresh- 
water Tertiary strata, both east and west. of the great ‘‘divide.’? At the Moqnis villages the 
strata f forming the table-lands begin to rise toward the east, and near Fort Defiance. they 
show the disturbing influence of the most, westerly axis of elevation of. the Rocky 
| tain system. Further east, to and bey ond the Rio Grande, they are much dislocated, and 
finally lose. their distinctive character i in the intricacies. of the mountain ranges. © 
