The chief points in the selection of Portland stone for building purposes depend upon the purpose for which the stone is required. There are four distinct kinds, of which three are usually sent into the market. The best is True Roach, 2 ft. or 3 ft. thick, consisting of a mass of fossils united by a cement composed of carbonate of lime, distinguished from Bastard Roach by its containing the Portland screw fossil; it is much used in engineering works. The Whitbed is the most useful Portland stone, consisting of fine oolitic grains, well cemented together, with a small amount of shelly matter at intervals. It is a good weathering stone, will take a fine surface and a sharp arris, and is used for the finest ashlar work. Basebed is very similar in appearance to Whitbed, but of a less roe-like texture when examined through a magnifying glass, and more free from shelly matter. Being more uniform in texture and softer to work, it is preferred by masons, but does not weather so well. It is useful for internal work and carving, and is generally known as "best-bed." All stone should be laid on its natural bed, but in Portland stone it is not so easy to detect this as in more laminated stones.