SOILS AND SUBSOILS OF SYDNEY AND SUBURBS. 221 



for man. Pure water and pure air are essential to the 

 maintenance of a proper standard of health and the purity of 

 each depends mainly upon the condition of the soil. Authorities 

 charged with the duty of procuring a water supply for a centre 

 of population have always to pay particular attention to the state 

 of the soil on the proposed catchment area and such area rarely 

 includes the site where the population to be supplied is located. 

 It may therefore be said that the effect of unhealthy soils upon 

 the health of the residents thereon is chiefly transmitted through 

 the air. Gases evolved from decomposing organic matter render 

 the ground air impure. The presence of an excess of moisture in 

 the soil causes coldness and dampness, and the vapour constantly 

 rising from the surface is the vehicle for the dissemination in the 

 air of many deleterious matters and germs. 



It is not necessary at this stage to make a general statement 

 of the measures usually taken to remedy defects in the healthiness 

 of soils. These will be particularly referred to when considering 

 the soils of the urban and suburban districts of the metropolis to 

 which your attention will now be directed. 



The natural soils of the metropolitan area may be divided into 

 four classes in two groups. The first group comprises soils formed 

 in situ from the surface rocks. The two divisions of this group 

 correspond to the two characteristic rocks of the area, viz : first 

 the sandstones and second the clay shales. The second group 

 comprises soils formed from drift and waterborne material, and 

 the two divisions are first the drift sand beds, second the fluviatile 

 deposits. The relation of these divisions to each other will be 

 better understood after reference has been made to the geology of 

 this section of the country. The geological formation of the area 

 under review is sedimentary, consisting of massive horizontally 

 bedded sandstone strata, surmounted with a partial capping of 

 ferruginous clay shale beds. The physical features have been 

 developed by erosive action, which has produced in the sandstone 

 rugged and precipitous outlines, and in the clay shale areas gentle 

 and regular undulations. The sandstone attains its greatest 

 elevation northward and eastward of the city where it also is the 

 prevailing rock. Going westward from the city the sandstone is 

 less and less elevated, the clay shale capping which appears in 

 isolated patches on the sandstone summits around the city becomes 

 more extensive and increasing in area and thickness further west, 

 the sandstone is at last totally obscured. 



The clay shales and the sandstones form the surface strata of 

 the whole area, except where they are obscured by drift sands in 

 the vicinity of the coast, and by mud deposits along the courses 

 of the Parramatta and Cook's rivers and their tributaries. On 

 the sandstone areas, which as before stated, lie chiefly to the north 



