WarD.] PETRIFIED FORESTS OF ARIZONA. 327 
mit is a bed, some 20 feet thick, of coarse, gray, conglomeratic, cross- 
bedded sandstone, at many places in which were found, firmly embed- 
ded, logs and branches of the petrified wood, often projecting from it 
in the cliffs, and clearly in place. This, then, is the true source of the 
fossil wood, and after several days’ study on all sides of the area I 
became convinced that no other layer holds any of it, at least in this 
region. 
This bed was found at nearly all points where the requisite elevation 
can be attained, but the petrified iogs do not occur in the same abund- 
ance throughout. They are massed or collected together in groups or 
heaps at certain points and may be altogether absent at others. From 
their great abundance in the three areas above described, which may 
be called the upper, lower, and middle forest, respectively, but in all 
of which they are out of place and lie several hundred feet below 
their proper position, it must be inferred that the stratum which held 
them was especially rich and that the trunks must have lain in heaps 
upon one another. This bed may have been considerably thicker in 
these areas than it is farther out on the margins where it is now found 
in place. 2 
At only two points within the general petrified forest area did I find 
remnants of this bed which had not been broken down and disinte- 
grated. One of these is at the extreme northern end, half a mile 
northeast of the upper forest. Here there is a small mesa, which lies 
at an elevation of nearly 5,700 feet, or about 400 feet above the valley 
that contains the upper forest. It is isolated, and its nearly flat top, 
which is approximately circular, is about half a mile in diameter. 
The coarse conglomeratic sandstone stratum, 20 to 30 feet in thickness, 
occupies the summit of this mesa and is often hardened into rock, but 
in all essential respects it is identical with that of the elongated mesa 
on the southwest side of the a1ea above described. The petrified wood 
is less abundant here, but sufficiently common, and is embedded in and 
often projects from the sandstone ledges. 
Besides the fact that this bed lies wholly within the petrified forest~ 
area, there is another important circumstance which serves to give it 
special prominence. One of the most celebrated objects in this entire 
region is the well-known ‘‘ Natural Bridge,” mentioned by so many 
travelers, consisting of a great petrified trunk lying across a canyon 
and forming a natural footbridge, on which men may easily cross. 
This occurs on the northeast side of the above-mentioned mesa, near 
its rim, and the bed in which it lies is the coarse sandstone which holds 
all the petrified wood. The Natural Bridge therefore possesses the 
added interest of being in piace, which can be said of very few of the 
other petrified logs of this region. 
It was observed in the southwestern exposure and at other points 
that all ‘the petrified logs and blocks lying in the sandstone or only 
