The United States 169 
Some of the trunks have a length 
of lOO to 200 feet, and a thick- 
ness of seven to ten feet. Even 
in prehistoric times, stone-hammers, 
arrow-heads, and knives were made 
of the petrified wood, whose remains 
are still to be found. Again, old 
Indian houses are built of this beauti- 
ful material. In recent times, the 
material has been exploited by indus- 
try ; and several companies on the spot 
manufactured objects of art and orna- 
ments from it. A great many of the 
latter were sold at the Paris Exhibition, 
in 1900. Thus the existence of the 
silicified forest was endangered, and a 
Public Reserve was established, on the 
suggestion of the State of Arizona. 
Associations. There are many 
scientific and other societies working 
at the investigation and protection 
of natural monuments in the U.S.A. 
The American Scenic and Historic 
