298 PETRIFIED FORESTS OF ARIZONA. 



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 Mex 

 ico 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 several of these transverse cracks, 

 and all the good photographic views of it show them. I counted four, 

 but most of them seem to be as yet only partial and do not probably 

 extend entii'ely through the trunk. There is one, however, near the 

 left bank of the canyon which has the appearance of doing so, and the 

 trunk is probably only kept from parting at this point by the mechan- 

 ical adjustment which causes the adjacent faces to perform the office 

 of a keystone to an arch. Any considerable shrinkage due to climatic 

 or other causes would overcome this influence and the entire bridge 

 would crash to the bottom of the canyon and roll down the escarpment 

 in a number of huge segments. 



An examination of the relations of the Natural Bridge to the gulch 

 which it spans shows clearly that the trunk was primarily entombed 

 in the sandstone bed covering this entire region, and that, with the 

 progress of erosion which ultimately carried away the entire plain to 

 the north, as well as in other directions, leaving this small mesa, it was 

 at last exposed, and lay for a great period near the rim of the escarp- 

 ment. At first it was only partially ))uried and later came to lie on 

 the surface of the ground. As the land rises somewhat to the south 

 of it, rills were formed above, and in times of floods or heav}^ rain it 

 obstructed the flow of the water, forming a sort of dam. The water 

 lying against it long after it had ceased to overflow it, tended to disin- 

 tegrate the rock upon which it lay, until eventually it found its way 

 through beneath it at some one point. The smallest opening of this 

 nature would soon become a free passage for the water, and a simple 

 continuation of this process of local erosion would ultimately result in 

 the formation of the entire gorge as it exists to-day. 



The other case which I observed of the presence of the conglomeratic 

 sandstone within the general petrified forest area occurs near its center, 

 about midway between the upper and lower forests along the narrow 

 portion of the valley of the creek described above, on both sides of the 

 canyon and near the level of its bed, at an altitude of about 5,300 feet. 

 The exposure was typical in all respects, and logs were seen projecting 

 from the canyon walls, from one of which specimens were collected. As 

 this exposure is 400 feet below that in which the Natural Bridge occurs 

 and 450 feet below that on the southwestern mesa, its presence there 

 can be accounted for only on one of two hypotheses, either that of the 



