These foraminifera are so very small that numbers of them can live in a single drop of water! Yet, strange to say, all the chalk in the world is made of their shells! For in days of old--thousands and thousands of years ago--they were found in the sea in millions of millions of millions. And as they died their empty shells sank down to the bottom of the sea in such enormous numbers that at last they formed a layer hundreds of feet thick. Then suddenly one day there came a great earthquake, and a great deal of this vast layer of shells was forced up above the surface in the form of what we now call chalk. So that “the chalk cliffs of old England” are really made of nothing but shells, so very small indeed that you cannot see them without the help of a very strong microscope! There are a great many different kinds of foraminifera. But if you look at them through a good microscope you will always see that their shells are pierced by the tiny holes from which they take their name. CHAPTER XI SEA-WEEDS PLATE XLV THE BLADDER-WRACK (1) I dare say that you would like to know something about the sea-weeds which you may find on the shore; so I am now going to describe some of those which you are almost certain to meet with. First of all, then, and commonest of all, there is the bladder-wrack. Wherever there are rocks on which it can grow you will always see it in great masses. And after every storm enormous quantities of it are torn off and flung upon the beach. Then the farmers send down their carts to carry it away. For after it has been piled up in heaps for some time, so as to allow it partly to decay, it makes a most useful manure; and the farmers are only too glad to be able to spread it over their fields. This plant is called the “bladder-wrack” because of the odd little oval bladders filled with air which are found in the leaves, and which explode with a slight report if you tread upon them or squeeze them. [Illustration: PLATE XLV 1. THE BLADDER-WRACK. 2. THE OAR WEED.] PLATE XLV THE OAR WEED (2) This is a very fine sea-weed indeed, for it often grows to a height of ten or eleven feet. But you are not likely to see it growing, for it lives in rather deep water, where it is always covered even at the lowest tides. It is often flung up by the waves, however, and you must many times have noticed its long, thick stem and flat plate-like leaves lying upon the shore as the tide was going down. The stem of the oar weed is often used for making the handles of knives. When it is quite fresh, it is so soft that the “tang” of a knife-blade--the part, that is, which is fastened into the handle--can be forced into it quite easily. But if it is put aside for a few months to dry it becomes as hard and solid as horn, and holds the blade so firmly that it is almost impossible to pull it out again.