After the Middle Old Red Sandstone had stood for many ages as dry land, the eroding action of the atmosphere, rain, and streams gradually eliminated all inequalities and produced a nearly level surface, upon which by degrees a new lake was formed, occupying a large part of Orkney and Caithness. In this lake sand accumulated, which now forms the highest hills of Orkney. The yellow sandstones of the Hoy and Walls hills are deposits of this lake, and belong to the Upper Old Red Sandstone, which is not represented in the other islands of the archipelago, but reappears at Dunnet Head on the south side of the Pentland Firth. Flagstone is a material that yields readily to the influence of the weather, and the subsoil formed is a rubble consisting of loose fragments of rock embedded in a brown clay formed by the softer and more weathered portion of the underlying rock. This gives a well-drained soil, and as the flagstones always contain lime, potash, and phosphates, and are frequently mixed with sand, the resultant soil may be generally described as a clayey loam of moderate fertility. Soil of this character covers a large part of the flagstone formation of Orkney, especially where the land is of moderate elevation, and surface accumulations of boulder-clay, or alluvium are at a minimum. At higher elevations on the other hand the character of the flagstone soils is frequently modified by the presence of peat. The soils of the sandstone formation, which occur in parts of South Ronaldshay and Hoy, and to a larger extent in Burray and Eday, are of inferior fertility, partly on account of their more porous nature. In Burray and Eday a large portion of the sandstone districts are in consequence left uncultivated, and utilised only as rough pasture. The same is true of the soils of the Upper Old Red Sandstone formation, which covers by far the larger portion of the parishes of Hoy and Walls, but most of this land is, in any case, above the altitude to which cultivation is usually carried in Orkney. Of “drifts,” or loose surface deposits, the boulder-clay is the only one that materially affects the agricultural quality of the soils of the Islands as a whole. The boulder-clay of Orkney, which on account of the particular direction—northwestwards over the Islands from the North Sea—taken by the ice at the period when the archipelago was subjected to glaciation, contains a considerable additional quantity of lime in the form of shells scraped up from the bed of the ocean, overlies the rock formations in the lower grounds over a large portion of the county. This deposit, which varies in thickness from less than a foot up to forty feet or more, has been calculated to occupy at least one-third of the area of the Islands, and it provides the most fertile soil that the county contains, particularly where the land has been improved by long cultivation and artificial drainage. 6. Natural History