There is quite a variety of other types of preservation among fossil plants, but they are of minor interest and importance, and hardly justify detailed consideration. One example that should be mentioned is Amber. This is the gum of old resinous trees, and is a well-known substance which may rank as a “fossil”. Jet, too, is formed from plants, while coal is so important that the whole of the next chapter will be devoted to its consideration. Even the black lead of pencils possibly represents plants that were once alive on this globe. Though such remains tell us of the existence of plants at the place they were found at a known period in the past, yet they tell very little about the actual structure of the plants themselves, and therefore very little that is of real use to the botanist. Fortunately, however, there are fossils which preserve every cell of the plant tissues, each one perfect, distended as in life, and yet replaced by stone so as to be hard and to allow of the preparation of thin sections which can be studied with the microscope. These are the vegetable fossils which are of prime importance to the botanist and the scientific enquirer into the evolution of plants. Such specimens are commonly known as Petrifactions. Sometimes small isolated stumps of wood or branches have been completely permeated by silica, which replaces the cell walls and completely preserves and hardens the tissues. This silicified wood is found in a number of different beds of rock, and may be seen washed out on the shore in Yorkshire, Sutherland, and other places where such rocks occur. When such a block is cut and polished the annual rings and all the fine structure or “grain” of the wood become as apparent as in recent wood. From these fossils, too, microscopic sections can be cut, and then the individual wood cells can be studied almost as well as those of living trees. A particularly notable example of fossil tree trunks is the Tertiary forest of the Yellowstone Park. Here the petrified trunks are weathered out and stand together much as they must have stood when alive; they are of course bereft of their foliage branches. Such specimens, however, are usually only isolated blocks of wood, often fragments from large stumps which show nothing but the rings of late-formed wood. It is impossible to connect them with the impressions of leaves or fruits in most cases, so that of the plants they represent we know only the anatomical structure of the secondary wood and nothing of the foliage or general appearance of the plant as a whole. Hence these specimens also give a very partial representation of the plants to which they belonged.