Of these leaf-prints the finest specimens collected in the Bad Lands come from strata which have been baked by the burning coal. This burned material furnishes a matrix of sufficient hardness to preserve perfectly the mold and to endure the stroke of the hammer which brings to light the hidden image, and so the life-history of Dakota, like the history of some of the old Oriental monarchies, is revealed by the cleaving of burned bricks.
So far as I am informed, no systematic search for these fossil leaves has ever been made. They occur on the surface in isolated spots, and different localities furnish different as well as similar forms. The baking to which the fossil-bearing beds have been subjected has, in a measure, obliterated the distinction of strata, so that it is difficult in any case to determine the exact horizon, or to say whether all the leaves are from about the same level, and hence contemporaneous; it suffices our purpose to know that they are nearly so. At all events, in strata such as these, and as geology reckons time, no intervals have been very great, and we may omit discussion of the relative age of the leaves, and consider immediately their kinds and meaning. We have represented, in Figs. 3-12, leaves of the following genera:Platanus, Populus, Juglans, Corylus, Carpinus, Persea, Mens, Sequoia, Comics These names are all familiar, although we are not accustomed to see them grouped together. Platanus is represented throughout the northern Mississippi Valley by the sycamore, frequenting the water-courses and rocky banks, and often attaining grand dimensions. Two species of the genus occur in California, two in Mexico, and one in the far Levant. Populus we know from our aspens, balm of Gilead, and more than all by the cottonwood—a prairie-tree—abundant along our Western rivers, and following the Missouri and its tributaries to the very foot-hills of the Rocky Mountains. These trees all secrete about their buds more or less of fragrant wax, and possibly from the tiny pits seen at the base of the leaf of P. glandulifera exuded some such balsamic gum which spread and polished the upper surface of the young leaves. Of Corylus and Carpinus little need be said. The hazels and hornbeams are sufficiently well known as characteristic of north temperate forests everywhere. The genus Juglans we know from our invaluable walnut, once common throughout the Eastern United States. A single species is found also in Asia Minor and Europe. Cornus, the dogwood, has some northern species. But the three remaining genera are more interesting. Persea is a laurel, and laurels are especially tropical plants, extending in hardier forms as evergreens into the sheltered or milder parts of the temperate regions. This particular genus extends along the Atlantic coast from Delaware southward, and is abundant in the West Indies. Ficus is also a tropical genus, or, at least, occurs in warm climates only, as in Florida, South America, around the Medi-
- For the identification of these leaves, except one or two, I am indebted to Professor Lesquereux.