328 OLDER MESOZOIC FLORAS OF UNITED STATES. 
recently washed out of it are surrounded by a coating of the sandstone 
firmly cemented to the exterior. The absence of this coating from 
most of those in the principal forests is due to their long exposure to 
climatic influences, which ultimately disintegrate and detach the sand- 
rock adhering to them and strip them clean to the body of the trunks 
themselves. That this process requires ages of time is proved by the 
fact that the Natural Bridge is still coated over a large part of its 
surface by the remains of the cemented sand rock in which it was once 
completely embedded. This is true chiefly of the lower portion, and 
farther up the trunk it has nearly all disappeared. The trunk is in an 
excellent state of preservation and is complete to the base, where it is 
abruptly enlarged and shows the manner in which the roots were 
attached. This portion still lies partially buried in the sandstone, 
which is the same in character as that which still adheres to the lower 
20 feet. The canyon or gulch has a due north direction and is very 
precipitous, beginning only 200 yards above the bridge and rapidly 
broadening in its descent. At the point where the bridge crosses it is 
about 30 feet wide, but the trunk lies diagonally across and measures 
44 feet between the points at which it rests on the sides of the canyon. 
The angle is nearly 45°, and the tree lies with its roots td the south- 
east and its top to the northwest. The canyon is here about 20 feet 
deep, and from its bottom and slopes several small trees are growing, 
some of which rise considerably aboye the bridge. The trees are 
mostly cedars, but there is one cottonwood (Populus angustifolia). 
The root is quite near the brink of the canyon, but rests on a solid 
ledge for a distance of 4 feet, so that there is no probability that in 
this dry region it will be endangered by further erosion. The total 
length exposed is 111 feet, so that more than 60 feet of the upper part 
lie out on the left bank of the canyon. At about the middle of the 
canyon, and above where the coating of sandstone still adheres, it 
measures 10 feet in circumference, giving a diameter of over 8 feet. 
At the base it is now 4 feet in diameter, but the thickness of the incrus- 
tation is not exactly known. At the extreme summit the diameter is 
reduced to 18 inches. As in the case of practically all the petrified 
logs of the region, there are no indications of limbs or branches at the 
top. The significance of this fact will be noted later. 
A conspicuous characteristic of all the petrified trunks, not only of 
this area and of the general Triassic terrane of Arizona and New 
Mexico, but of all petrified forests, is their tendency to break across 
into sections or blocks of greater or less length. All travelers have 
remarked this, and the sketches given by Mollhausen and in the Pacific 
Railroad reports show them thus divided. Some observers have noted 
the fact that the Natural Bridge has severaLwot these transverse cracks, 
and all the good photographic views of it show them. I counted four, 
