In the first place, petrification is the name we give to the animal and vegetable bodies which have, by slow process, been converted into stone. We mean very much the same thing when we refer to “Fossil Forests.” Although in most instances there are comparatively few traces of its vegetable origin left, coal owes its existence primarily to the vast masses of vegetable matter deposited through the luxuriant growth of plants in former epochs of the earth’s history, and since slowly converted into a petrified state. Coal fields today present abundant indications of the existence of huge ancient forests, usually in the form of coal formed from the roots of the trees. Several such forests have been uncovered, of which one in Nova Scotia is a good example, remains of trees having been found there, six to eight feet high, one tree even measuring twenty-five feet in height and four feet in diameter. The remains of a fossil forest have been found in an upright position in France, and in a colliery in England, in a space of about one-quarter of an acre, there have been found the fossilized stumps of seventy-three trees, with roots attached, and broken-off trunks lying about, one of them thirty feet long and all of them turned into coal. A remarkable group of petrified trees, some of them twelve feet in diameter, exists in California, and another in Yellowstone Park, in which the trees are still erect, though converted into stone. An extraordinary forest of such trees has been found in Arizona, lying over a wide space of ground, some of them six feet in diameter and perfectly preserved. These trees are rather mineralized than fossilized. They are found in volcanic regions and are supposed to be due to the action of hot water, which carried off the organic material and deposited dissolved silica in its place. In some instances the wood has been converted into solid jasper or has been changed into opal or agate, or filled with chalcedony or crystallized quartz, with beautifully variegated colors. [Illustration: TREES THAT HAVE TURNED TO STONE A scene in one of the Petrified Forests of Arizona. Broken trunks of trees are lying all about.] What Animals are the Best Architects?