The chief trees of the country are the aspen (Populus tremuloides), the ash-leaved maple (Negundo aceroides), oak (Quercus alba), elm (Ulmus Americana), and many varieties of willow.
The iron tree (Parrotia persica), the silk acacia, Carpinus betulus, Quercus iberica, the box tree and the walnut flourish freely, as well as the sumach, the pomegranate, and the Gleditschia caspica.
In these islands, we find forests i or woods of oak (Quercus Robur and Q.
Torreya, now confined to North America and Japan, still lingered,- as did Ocotea, now profusely developed in the tropics, but in north temperate regions only existing in the Canaries: the evergreen oaks, so characteristic of the Miocene, were reduced to the existing Quercus hex.
Quercus Robur has left no trace in the Tertiary deposits of Europe and it is most nearly all~d to east Asiatic species.