formed its centre, also split into two, and standing in high relief, with its scales showing clearly. Similar nodules or balls of clay are found to-day, forming in slowly running water, and it may be generally observed that they collect round some rubbish, shell, or plant fragment. These nodules are particularly well seen nowadays in the mouth of the Clyde, where they are formed with great rapidity.

Fig. 9.—Clay Nodule split open, showing the two halves of the cone which was its centre. (Photo.)

Another kind of preservation is that which coats over the whole plant surface with mineral matter, which hardens, and thus preserves the form of the plant. This process can be observed going on to-day in the neighbourhood of hot volcanic streams where the water is heavily charged with minerals. In most cases such fossils have proved of little importance to science, though there are some interesting specimens in the French museums which have not yet been fully examined. A noteworthy fossil of this type is the Chara, which, growing in masses together, has sometimes been preserved in this way in large quantities, indicating the existence of an ancient pond in the locality.

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.

Fortunately, however, there is still another type of preservation of fossils, a type more perfect than any of the others and sometimes combining the advantages of all of them. This is the special type of petrifaction which includes, not a single piece of wood, but a whole mass of vegetation consisting of fragments of stems, roots, leaves, and even seeds, sometimes all together. These petrifactions are those of masses of forest débris which were lying as they dropped from the trees, or had drifted together as such fragments do. The plant tissues in such masses are preserved so that the most delicate soft tissue cells are perfect, and in many cases the sections are so distinct that one might well be deluded into the belief that it is a living plant at which one looks.

Very important and well-known specimens have been found in France and described by the French palæobotanists. As a rule these specimens are preserved in silica, and are found now in irregular masses of the nature of chert. Of still greater importance, however, owing partly to their greater abundance and partly to the quantity of scientific work that has been done on them, are the masses of stone found in the English coal seams and commonly called “coal balls”.

The “coal balls” are best known from Lancashire and Yorkshire, where they are extremely common in some of the mines, but they also occur in Westphalia and other places on the Continent.