Agatized Rainbows: A Story of the Petrified Forest — Inside the Classic

ess. Bit by bit, water removed wood tissue and in its place left a mineral deposit in exactly the same form as the original, so that when the process had been completed there was no wood left but in its place an exact stone duplicate. This theory was accepted for a very long time, but recently some scientists were not satisfied with it because certain chemical actions that would have to occur during such a process were difficult to explain.

Just prior to 1940, several scientists investigated the process, and from their findings decided that the wood was not petrified by _replacement_ but by the _infiltration_ of mineral-bearing water into the wood and the deposition of this mineral in the air spaces within the wood tissue. This process, they believe, continued until all of the microscopic spaces in the wood were filled solid with this deposit and the petrified log, composed of 98 per cent by volume of mineral deposit and 2 per cent cellulose and lignin wood tissue, was the result. The original wood tissue acted, they think, as a framework to hold the mineral deposit like the walls of a building would act if the rooms and spaces between the walls were filled in solid with liquid concrete. This accounts for retention of the cell structure, annual rings, and other features of the original wood. The petrification of wood has never been studied sufficiently, and there are many questions for which satisfactory answers have not yet been advanced.

Although woods in different localities have been petrified by other minerals, the most common is silica. In the case of this petrified wood, the silica was deposited in an agatized non-crystalline form. The normal color of the silica without mineral stain is a white or gray. Sometimes small amounts of other minerals were in the solution along with silica, or in some cases were brought in during the millions of years of burial as a secondary deposit in the cracks, checks, or other openings in the petrified or partially petrified wood. Iron oxides in small quantities produced the great variety of shades of red, brown, and yellow. The black color in most cases is due to manganese oxide or carbon. Thus, the combination of minerals produced a rainbow of colors in the agatized wood.

Whenever there were small checked places, cracks, or hollows in the wood, we find that they are often either filled or lined with quartz crystals or occasionally with amethyst crystals.

The term “chalcedony” (pronounced kal-sed´-nee) is a broad one usually applied to any compact mass of silica free of definite color pattern, but it is also frequently used to describe all forms of silica whether translucent or opaque, and regardless of color. Agate, therefore, may be considered a variegated chalcedony. Agate is translucent and has a definite color or pattern. Jasper is opaque and may be either red, brown, yellow, blue, or green in color. Quartz minerals are harder than most types of steel, and there are only about 30 other minerals that exceed it in hardness. In the mineral scale of hardness, quartz is rated at 7 and diamond, the hardest of all, at 10. Petrified wood weighs about 166 pounds per cubic foot.

“Who sawed these trees” is one of the questions visitors frequently ask. It is a natural query because most of the logs are cracked into sections, in many instances of rather uniform length, and each broken face is smooth enough to appear almost like a saw cut. At first glance this does give the impression that someone, possibly a Paul Bunyan with an enormous diamond-toothed saw, had cut the logs into “stove wood” lengths. Although there may be some differences of opinion about how this fracturing occurred, the present explanation by scientists is that most of this breakage took place during the period of uplift of this section of the country. The gradual movement and elevation of the earth’s crust caused numerous earthquakes. The shock waves of the tremblor traveling through the earth set up vibrations which caused the deeply buried, brittle, petrified logs to crack. The harmonic vibrations created by the rhythm of the regular shock waves caused the cracks to be rather regularly spaced. At first these cracks were tiny, but centuries later, after the logs were exposed on the surface, the weathering actions and the shifting and settling of the soil beneath them caused the cracks to widen and eventually the fractured sections separated. Occasionally breakage may also occur when soil washes out from under one end of a log and its weight causes it to sag and crack. The normal fracture line of this material is at right angles to the lineal axis, and the rather smooth face causes the broken surface to appear much like a saw cut.