SECTION OF STRATA AT LAGUNA. 97 
East of Agua Azul the strata dip very rapidly in that direction, and the walls enclosing the 
valley are composed of the yellow Cretaceous sandstones, on the north side covered with a thick 
sheet of trap. On the south side the Cretaceous strata form detached mesas, which are without 
the trap covering, but show in many places fractures and dislocations. For many miles the 
bottom of the valley is occupied by a lava stream, evidently of more modern date than any of 
the erosions of the surface. It has dispossessed the watercourse of its bed, and, following all 
its sinuosities, now lies a congealed flood, as black, bare, and ragged as if just poured out. 
These geological features continue without change to Covéro and Laguna-—Cretaceous sand- 
stones composing the walls of the valley, which is formed entirely by erosion. The northern 
wall is capped with trap. At Covéro the greenish shales, enclosed in the yellow sandstones, 
- contain large numbers of Gryphaea Pitcheri. The town is built upon a stratum of coarse yellow 
sandstone, which here forms the bottom of the valley, and is cafioned by the stream. 
Laguna is built upon a similar stratum, lower in the series, and separated from that of Covéro 
by white and bluish shales. 
At Laguna the Rio San José cuts through the Cretaceous formation, and for several miles 
below flows in a picturesque cafion, which it has excavated in the red and white calcareous sand- 
stones, gypsum, &c., of the Marl series. 
The section of the strata exposed at Laguna is as follows, the thickness being estimated: 
1. Yellow sandstones and greenish shales, Cretaceous-----+ +--+ ++++eeee ceeees 250 feet. 
2. White and bluish shales, Cretaceous +++ ++ sere se este es renee cece ee eeeees 100 feet. 
3. Yellow sandstone, Cretaceous +0. a2 os 6: <3 eee aie 658 hs ee toe. ead » ces re ox iecacg: ave 50 feet. 
4. Soft red and white calcareous sandstones, (in cafion east of Laguna,) variegated 
Marl series: « << <6 68 Sb Whines Cote ASF On be ee wie e's Oe Oe: 0s tw weet en betes 150 feet 
‘si Gypsum, variegated Watt IBOTIDO eso Sas G68 aes sd ew sle oA DE be ene ein 80 feet. 
6. Blue calcareous shale, variegated Marl series -+--++ ++eeees eee ee cree eee 20 feet. 
7. White soft calcareous sandstone, variegated Marl series -----+---+++++++-+- 40 feet. 
8. Red soft calcareous sandstone, Salt group ---+ +++ ests tees eee e ee teen wees 120 feet. 
9. Red and green foliated sandstones and blood-red shales, Salt group. 
From Sheep Spring to the Puerco the strata are very much broken up and displaced. The 
upper member of the saliferous sandstone group is beautifully exposed at Sheep Spring, and here 
exhibits precisely the same characters as in the localities where it was noticed in the Navajo 
country. 
This formation soon disappears with an easterly dip, and for a short interval the surface is 
strewed with silicified wood derived from the lower portion of the variegated marls. Thence 
eastward nothing is visible but the Cretaceous sandstones and green shales, forming many 
detached mesas, in which the strata are broken and inclined at various angles. 
Between the Puerco and the Rio Grande we were almost constantly crossing hills of loose 
sand, which entirely conceal the underlying rocks. 
On the east bank of the Puerco the green shales, so conspicuous immediately west of it, are 
visible for a short distance, when they give place to white, soft, concretionary, tufaceous strata, 
in which I was unable to discover fossils or any indications of their age. In their lithological 
character these beds differ widely from the Cretaceous rocks gn which they rest, as they also do 
from any strata containing Cretaceous fossils met with on our route. They much more resemble 
the fresh water Tertiary beds of the Arkansas, White river, &c., and, guided by this similarity, 
I should be inclined to consider them of that age. Their physical characters are not of great 
value, however, and it is possible they are Cretaceous. If belonging to the latter formation, I 
think it will be found that they are more recent than any of the Lower Cretaceous strata described 
in the preceding pages, and should be placed in the upper division of that series. 
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