The upper part of the limestone is shaly and argillaceous, and rests below a series of shales and thin sandstones. If the question were asked, what caused the change from limestone to shale, from the deposition of a calcareous to that of a muddy sediment, several answers might be given. The most probable seems to be the following. The limestone on weathered surfaces displays the mouldering casts of cypris-cases sometimes in such abundance as to show that the rock must be largely made up of them. The cyprides of the present day probably cast their shells annually; the integuments thus thrown off forming under favourable circumstances a thin mouldering calcareous marl at the bottom of the pond or marsh, along with the decaying shells of _paludina_, _planorbis_, _limnea_, or other fresh-water molluscs. We may conceive the Burdiehouse limestone to have had a similar origin. The cyprides, inhabiting water that contained little argillaceous matter, must have propagated by myriads, and during a long period of repose, in which the conditions of land and sea, and the directions of tidal currents and river-courses, appear not to have greatly varied in the neighbourhood of Burdiehouse, the calcareous exuviæ of these minute animals, along perhaps with the remains of other estuarine or fluviatile organisms,[60] would form each year a scarce appreciable stratum, until by slow aggregation a bed twenty-seven feet deep was elaborated. Each successive annual layer would hardly settle down more perceptibly or more rapidly than "the flickering dust that mottles the floor of some old haunted chamber." [Footnote 60: Though I have never observed molluscan remains in the limestone of Burdiehouse, they are abundant twelve miles to the west, in the equivalent strata around Mid-Calder, one little gastropod being especially plentiful near the base of the calcareous rock in a seam known to the quarrymen as the "Buckie fake." I have not met with specimens sufficiently perfect for identification, the hard splintery nature of the rock seldom allowing anything but a cross-section to be seen save on weathered specimens, where the general contour of the shells has sometimes reminded me of _Paludina multiformis_ grouped together in a recent fresh-water marl. In the shales above the Burdiehouse limestone, Dr Hibbert states he found a _unio_ (?), called by him _U. nuciformis_. _Trans. Roy. Soc. Edin._ vol. xiii. p. 245.]