Page:Popular Science Monthly Volume 28.djvu/374

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NDOUBTEDLY one of the greatest of American wonders is the silicified forest in Arizona, known as Chalcedony Park—a park only in name, however, for the giant trees which once grew there have long since fallen and silicified into agate and jasper. It is situated eight miles south of Corriza, a station on the Atlantic and Pacific Railroad, in Apache County, Arizona, twenty-four miles south-east of Holbrook. This marvelous deposit of probably a million tons of silicified trees covers a thousand acres. The wood is generally found projecting from the volcanic ash and lava, which is covered with sandstone to the depth of from twenty to thirty feet, and lies exposed in the gulches and basins where the water has worn away the sandstone.

The silicification probably took place in the following manner: The trees were overthrown and covered with volcanic ashes and tufa, the heated silicified waters, either gushing from springs or forced up by the violent volcanic action which felled the trees, percolated through the ashes, cooled on reaching the tree-level, and thus produced conditions favorable to silicification.

The moisture in the tufa may have effected a partial alteration, as also any waters that may have filtrated through it from rains or springs either hot or cold. Under these circumstances decomposition would be assisted and much silica be set free. The waters would become charged with this, the silica being held partly in solution similar to that in liquid glass, the silicate of soda of commerce. The silicious water then slowly penetrated the wood buried in the tufa and was slowly deposited in the cells of the wood. In this manner the fibers of the wood were replaced by the silica. The process was evidently a slow one, and the trees, from all appearances, were partly decayed and water-logged when the silicification took place. The jasper and agate generally replaced the cell-walls and fibers, and the transparent quartz filled the cells and interstices, especially where the structure was broken down by decay. These cell-centers and cavities produced the conditions favorable not only for the deposition of the silica as quartz, but also for the formation of the drusy, crystalline cavities of quartz and amethyst that enhance the beauty of the material so much. It is evident, from the rich variety of colors, that the waters held oxides of iron and perhaps manganese, as well as silica, the red color being caused by hematite, the yellows and browns by limonite, and the black by oxides of manganese.

It is possible also that the ash was deposited partly in water and thus heated it. There is every indication that the deposit is of