64 GEOLOGY. 
mesa at Camp 69, are here beautifully exposed, forming the most striking features in the wild 
and peculiar scenery of this and the neighboring cafions. 
The bottoms of these chasms and a portion of their bounding walls are colored by them to a 
deep blood-red, which contrasts strongly with the snowy white of the enclosed gypsum, and 
the yellow, white, green, &c., of the overlying rocks. As is almost universally the case with 
gypsiferous rocks, these strata are without fossils. 
‘The companionship of peroxide of iron and sulphate of lime, in the great number of localities, 
and distinct formations in which they occur together, is, as it seems to me, too marked to be 
accidental. This conviction has forced itself upon me in the examination of the extensive 
gypsiferous deposits of different ages which form so marked a feature in the geology of the 
country traversed by our party; and I have been led to look to the law of that relationship as 
probably affording a clue to the origin of all the great masses of gypsum which we observed. 
Without an exception, the red sandstones, clays, marls, limestones, &c., of whatever age, so 
common in New Mexico, contain notable quantities of gypsum, and it is also true that very 
little of this mineral is found associated with any other materials. 
Deferring for the present the discussion of this question in its details, I will only say here 
that the theory of the formation of gypsum which regards it as the result of the action of 
sulphurous waters or vapors on coral reefs or other masses of carbonate of lime, however 
applicable to other countries, is inadequate to explain all the phenomena of the gypsum beds 
of the Far West, because, Ist. They are too extensive to be the result of an action necessarily 
local. 2d. The gypsum is more generally and uniformly diffused through the rocks in which 
it occurs than is accordant with that hypothesis. 3d. The veins of gypsum which traverse the 
different strata present difficulties which, on that supposition, have seemed to me to be insur- 
mountable. They are evidently as much due to segregation as are the crystals of selenite in 
the clay beds. By their displacement of the enclosing rocks during the process of crystalliza- 
tion they seem to be of comparatively recent origin. 4th. The oxide of iron, so constant in 
its attendance upon the gypsum, has not only no place in that theory of its origin, but is to 
some extent incompatible with it. 
A more satisfactory explanation of the phenomena, as it seems to me, is afforded by making 
the iron an important agent in the process. If a blue or gray calcareous rock, containing large 
quantities of pyrites, were subjected to the peculiar exposure of the strata of the table-lands, 
profoundly cut by the draining streams, and permeated age after age by meteoric water 
carrying carbonic acid and oxygen to the pyrites, the result would be precisely what we now 
find to have taken place. The bisulphuret of iron, taking another equivalent of oxygen, is 
converted into the protosulphate of iron; then, by double decomposition, from bicarbonate of 
lime and protosulphate of iron, we should have sulphate of lime (gypsum) and protocarbonate 
of iron, which last would soon obtain another equivalent of oxygen, and be changed into the 
peroxide. 
This I am disposed to regard as the true theory of the formation of the gypsum disseminated 
through rocks so deeply colored by peroxide of iron, though there are doubtless many and 
striking instances of its production by exhalations of sulphuretted hydrogen, or springs con- 
taining sulphuric acid, acting on carbonate of lime. Upon the theory suggested the gypsum 
would be altogether a secondary product, being formed not only after the deposition of the 
strata containing it, but also in great part subsequent to their elevation above the ocean level. 
The displacement of strata after their consolidation by masses and veins of gypsum indicates 
its modern date, whatever its origin may be. 
a The Carboniferous or Mountain limestone (No. 13) was only seen in the bottoms of the sub- 
cafions, which are deeply cut in the floors of the greater ones, and in which the streams, where 
they exist, now flow. No fossils were noticed in it. 
