108 GUIDEBOOK OF THE WESTERN UNITED STATES. 
are probably the Triassic descendants of the Permian progenitors of _ 
the araucarian family. Doubtless they grew in a near-by region 
and, after falling, drifted down a watercourse and lodged in some 
eddy or a sand bank. Later they were buried by sand and clay, 
finally to a depth of several thousand feet. The conversion to stone 
was effected by gradual replacement of the woody material by silica 
in the form called chalcedony, deposited by underground water. A 
small amount of iron oxides deposited at the same time has given the 
beautiful brown, yellow, and red tints which appear in much of the 
material. 
All the ‘‘forests” present objects of interest, but a visit to the first 
and second illustrates most of the features. In places the logs 
are scattered over the surface in large numbers. They vary in size 
and in length of the trunk sections; in most places the sections are 
in no regular order, but some of them lie in line very nearly in their 
original positions. In the first forest they are all out of place, having 
either rolled down from their original positions in a sandstone layer 
at a higher level, or been left on the ground as the clay or sand that 
once inclosed them was washed away. In the second and third 
forests the original log-bearing stratum may be seen, with many 
logs only partly uncovered by erosion. Some of the tree trunks 
are 6 feet in diameter and more than 100 feet in length, but most of 
them are about half these dimensions. In the first forest there is a 
fine trunk that forms a natural bridge over a small ravine, the water 
having first washed away the overlying clay and sand and then, 
following a crevice, worked out the channel underneath. The length 
of this log is 110 feet, diameter 4 feet at butt and 14 feet at top. 
(See Pl. XXI, A.) 
The petrified woods are beautiful objects for study. When thin 
slices are carefully ground down to a thickness of 0.003 inch or less 
and placed under the microscope they show perfectly the original 
wood structure, all the cells being distinct, though now they are 
replaced by chalcedony. By studying the sections, F. H. Knowlton 
has found that most of these araucarian trees were of the species 
Araucariorylon arizonicum, a tree now extinct. It is known to have 
lived at the same geologic time also in the east-central part of the 
United States, where the remains of some of its associates have also 
been found. These included other cone-bearing trees, tree ferns, 
eyeads, and gigantic horsetails, which indicate that, at that time, the 
rainfall was abundant. 
The entire area of the ‘‘forests” is included in the Government 
reservation, and visitors are prohibited from carrying away any 0 
the petrified wood or damaging the logs in any way. Petrified wood 
occurs In many other places in these same beds to the north and south, 
notably in an area 6 miles north of Adamana or 5 miles north of Aztec 
