SOILS AND SUBSOILS OF SYDNEY AND SUBURBS. 225 



-clown on winter evenings are a certain indication of this. The 

 •air in the bottom of the valleys on a calm winter's evening is very 

 cold and chilly, whilst on the upper parts of the slopes and on the 

 summits the air is warm. The change in passing from the lower 

 to the upper elevation at night time is frequently as marked as 

 the passage from a cold to a warm room. Although the summits 

 -of the hills may be favourable to health as regards drainage, the 

 influence of contiguous low wet ground in the valleys is a danger. 



The drift sand beds present two extremes. The summits of the 

 :sand hills and ridges present all the conditions necessary for a 

 healthy soil and subsoil, but the low lying flat areas being charged 

 with water and also containing in places a considerable amount 

 ■of peaty matter, present conditions quite the reverse. 



The fluviatile deposits present all the conditions appertaining 

 to an unhealthy area. They have one redeeming feature — the 

 •danger to health is so apparent that population avoids them. 



The natural condition of the soils having been considered in 

 relation to the health of residents it is now necessary to study the 

 •effects likely to be produced on the soil by the settlement of 

 population thereon. 



First in regard to the sandstone areas where porous soils prevail 

 contamination of the soil by house refuse &c, will be in a large 

 measure corrected by oxidation of the contaminating elements. 

 Underground waters will be very liable to contamination, hence 

 the necessity for underground tanks being rendered impervious. 



Second, the clay shales areas. The contamination of the soil 

 will be accumulative and progressive. The minimum effect will 

 occur on the summits, the maximum will be felt in the low lying 

 parts. 



Third, the drift sand area. The effect on the high parts will 

 be similar to that on the sandstone areas, whilst on the low lying 

 saturated parts the contamination will be progressive and 

 accumulative. 



Fourth, the mud flats. Residence on these areas should not be 

 allowed under any conditions or the result will ever be disastrous. 



It will not do to close these remarks without some indication 

 of the lines along which action should be taken to improve the 

 •condition of the unhealthy soils and subsoils. The most decided 

 improvement will be effected by drainage. Wet and moist land 

 particularly in the valley bottoms of the clay shale and drift sand 

 areas should be dried and derated by means of subsoil drains. 

 Buildings should be prevented from encroaching on the main lines 

 ■of drainage, which should always be bounded by reserves. There 



