3. =Limestone= and =Magnesian Limestone Rocks= resemble the last in possessing considerable slope, so that the water passes away quickly. The hard oolite is the best formation under this head, and magnesian limestone the worst. 4. =Chalk= is a healthy soil when unmixed with clay, and permeable. Goitre is not so common as in limestone districts. If the chalk be mixed with clay, it is often damp and cold. 5. =The Sandstones= are healthy, soil and air being dry. If mixed with clay, or if clay lie under a shallow layer of sand-rock, the site may be damp. The hard millstone grit is a healthy formation. 6. =Gravels= of any depth are healthy, unless they are water-logged, as near rivers. Then a house on impervious clay may be drier than one on gravel. 7. =Sands= are healthy when of considerable depth; they may be unhealthy when shallow, and lying on a clay basis; or when the ground water rises through them from ground at a higher level. 8. =Clay=, =Dense Marls=, and =Alluvial Soils generally=, are apt to be cold and damp. Water is retained in them, and is often very impure. Thorough drainage improves a clay soil, and a house on a clay soil may be so constructed, as not to be damp. 9. =Cultivated Soils= are not necessarily unhealthy; but 10. =Made Soils= are always to be carefully avoided, as sites for houses. The materials with which inequalities have been filled up are commonly the contents of dust-bins, or some other refuse. The gradual putrefaction of organic matters renders the air about the houses impure. Such soils require free subsoil drainage, in order to keep them dry. It appears that the organic matters in soil are gradually removed by oxidation and bacterial purification. At least three years should be allowed before any such site is built on. The following table places different geological formations in their order of healthiness for the purposes of a site (Parkes):—