222 SOILS AND SUBSOILS OF SYDNEY AND SUBURBS. 



and the east of the city, the soil is derived from the weathering 

 of the sandstone and as may naturally be inferred consists 

 largely of sand. On some of the summits however, there is an 

 admixture of clay and ironstone — the relics of former clay shale 

 beds which have been abraded and removed by seolian and pluvial 

 action. The soil being sandy is porous, and lying on surfaces 

 develoj^ed entirely by erosive action, is well drained. The area is 

 elevated and the slopes are steep. Fissures are common in the 

 sandstone rock. These intercept a portion of the surface water 

 which subsequently appears lower down on the hill sides in the 

 form of springs. These springs are not uncommon, and steps 

 should be taken when they occur in the vicinity of dwelling houses, 

 to intercept the water by means of properly constructed channels 

 and carry it clear of the foundations. As a rule there is no 

 subsoil on the sandstone areas. 



The clay shale beds which prevail as the surface strata westwards 

 from the city present features totally different from those which 

 characterize the sandstone area. 



The agency which has scooped out the deep gorges and cut 

 rugged outlines in the latter, has carved jxieasant valleys and 

 rolling hills in the more plastic material of the former. The 

 surface of the clay shales usually gives no indication of the 

 structure of the underlying strata. The ferruginous clay shales 

 are easily disintegrated, hence the beds are never found outcropping 

 at the surface. The disintegration of the shale is largely due to 

 two causes, first the oxidation of the iron and subsequent 

 segregation from its matrix in the presence of moisture, and second 

 by the action of the roots of plants. The action of the roots has 

 produced the greatest effect. Roots of trees and plants grow and 

 penetrate to a great distance underground in search of nourishment 

 and moisture, not only laterally but deep down into and below 

 the subsoil. The small fibrous rootlets insinuate themselves into 

 the finest cracks and growing larger, gradually and irresistibly 

 force the sides apart. Tree roots have great powers of upheaval. 

 Interesting examples of this are often seen on hard rocky land. 



The amount of earth displaced by the roots of an ordinary large 

 forest tree amounts to many cubic yards. When the tree or plant 

 dies the roots decay and earth falls in, other trees and plants 

 spring up around ; they grow to maturity and die ; and so on for 

 ages. In this manner many a hard compact stratum becomes 

 broken up and comminuted. The results of the long continued 

 mechanical action of the roots, combined with the chemical change 

 which has taken place in some of the constituents of the clay shale 

 by the infiltration of air and moisture from the surface, may be 

 seen in the sections exposed in the railway cuttings. A top layer 

 of mould will be observed gradually merging into a yellowish and 



