The practical details of excavation need not be discussed. Suffice it to say that all humus and vegetation ought to be removed from building-sites. This is an easy matter in many districts, where the surface-soil is only a few inches deep. In others it is not so easy, and in towns especially, where the level of the streets and buildings has been rising for centuries with accumulations of organic and other refuse, it may be a costly operation. Certainly there is not that imperative necessity to remove all humus or even "made" ground, if it is intended to cover the site with an absolutely impervious ground-layer, but the ground-layer must be impervious. But under any circumstances, soil which has been contaminated by excreta, or by leakage from drains and sewers, must be removed, and. if necessary, clean material deposited in its place.

A by-law of the London County Council, adopted in 1891, thus deals with "made" ground: - "No house, building, or other erection shall be erected upon or portion of any site which shall have been filed up or covered with any material impregnated or mixed with any faecal, animal, or vegetable matter, or which shall have been filed up or covered with dust, or slop, or other refuse, or in or upon which any such matter shall have been deposited, unless and until such matter or refuse shall have been properly removed, by excavation or otherwise, from such site. Any holes caused by such excavation must, • d for a basement or cellar, be filed in with hard brick ,or dry rubbish, or concrete or other material to be approved by the District Surveyor".

Frequently the lowest floor of a building is above the natural surface of the ground, and filling has to be adopted. Any clean gravel, sand, brick-rubbish, or loam will be satisfactory. Ashes are sometimes used - usually boiler-ashes, but objection is often taken to them because they harbour vermin. Coarse concrete, however, does the same. Ordinary house-refuse and road-sweepings, which contain all manner of garbage, must not be used. Whatever kind of filling is adopted, it should be well consolidated, either by ramming, or, in the case of sand, by wetting.