By showing that the expressions of emotions could thus be explained naturally, Darwin undermined the position taken up by Sir Charles Bell, that the muscles used in producing expression were created for this special end. In 1876 he re-commenced geological work, bringing out his previous works on “Volcanic Islands,” and on “South America,” as a single volume. In this year too he wrote (November 16th) a most interesting letter to James Geikie, offering an explanation of the large stones standing in an upright position in the drift of the south of England. He had noticed the same thing with the flints in the red clay left upon the chalk as a residuum after the action of solvent agencies on the latter. This position he explained was due to the movement following the slow subsidence of parts of the clay as the chalk beneath dissolved, the flints arranging themselves along the lines of least resistance. This suggested to him the view that the flints in the drift are to be explained by the subsidence, during the warmer climate which followed the glacial period, of alternate layers of snow and drift accumulated during the winters and summers respectively, of the cold period itself. This interesting view will, Geikie believes, come to be accepted as the truth. [Sidenote: WORK OF EARTH-WORMS.] The book upon “The Formation of Vegetable Mould Through the Action of Worms,” must be included among his geological works, although it contains a great many observations of deep zoological interest. It has been stated already that he wrote a paper on this subject for the Geological Society in 1838. In 1877 he studied the mode by which Roman remains gain their protective covering of mould; again towards the end of 1880 he began systematically to prepare the book, which was published on October 10th of the following year. It was extremely successful, 8,500 copies being sold in three years. This interesting work affords a good illustration of the tremendous results obtained, even in a moderate time, by an immense number of workers all using their powers in one direction. Each single earth-worm swallows earth in the excavation of its burrow and for the nutriment it contains, the waste material being ejected as “castings” at the surface, and as a lining to the burrow. But although the amount of earth thus swallowed by a single worm is not large, worms are so numerous that “the whole of the superficial mould ... has passed, and will again pass, every few years through the bodies of worms.” The result of this unceasing transport of the deeper mould to the surface is shown to be the burial of stones, either singly or in layers (as in paths), the covering and consequent protection of ancient buildings, and the preparation of soil for plants. In addition to this, the geological denuding agencies are assisted by the manner in which the deeper soil is brought into a position in which it is exposed to their action.