The transformation of the deserted cases of numberless minute insects into a constituent part of a solid rock, first formed at the bottom of a lake, then constituting the sides of deep valleys, and the tabular summits of lofty hills, is a phenomenon as striking as the vast reefs of coral constructed by the labours of minute polyps. We remember to have seen such _caddis-worms_, as they are called by fishermen, very abundant in the wooden troughs constructed by the late Dr. Sibthorp, for aquatic plants, in the botanic garden at Oxford, to the cases of which many small shells of the G. Planorbis Limnea and Cyclas were affixed, precisely in the same manner as in the fossil tubes of Auvergne; an incrusting spring, therefore, may, perhaps, be all that is wanting to reproduce, on the banks of the Isis or the Charwell, a rock similar in structure to that of the Limagne. Mr. Kirby, in his "Entomology," informs us, that these larvae ultimately change into a four-winged insect. If you are desirous to examine them in their aquatic state, "you have only, (he says) to place yourself by the side of a clear and shallow pool of water, and you cannot fail to observe at the bottom little oblong moving masses, resembling pieces of straw, wood, or even stone--of the larvae itself, nothing is to be seen but the head and six legs, by means of which it moves itself in the water, and drags after it the case in which the rest of the body is enclosed, and into which, on any alarm, it instantly retires. The construction of these habitations is very various. Some select four or five pieces of the leaves of grass, which they glue together into a shapely polygonal case; others employ portions of the stems of rushes, placed side by side, so as to form an elegant fluted cylinder; some arrange round them pieces of leaves like a spirally-rolled riband; other species construct houses which may be called alive, forming them of the shells of various aquatic snails of different kinds and sizes, even while inhabited, all of which are immovably fixed to them, and dragged about at pleasure. However various may be the form of the case externally, within it is usually cylindrical and lined with silk."--_Introduction to Entomology, by Kirby and Spence._ _Engraving on Glass._