SOILS AND SUBSOILS OF SYDNEY AND SUBUEBS. 223 



reddish semi-plastic clay containing Hat stones ; with increasing- 

 depth the clay becomes more compact and dense until by almost 

 imperceptible gradations it passes into a laminated clay shale of a 

 uniform dark slaty colour and texture. The mould is usually 

 from six inches to a foot in thickness. The semi-plastic layer of 

 clay varies from one or two feet, to over six feet in thickness. It 

 is important to observe that the minimum depth of the clay occurs 

 on the summits of the hills, whilst the maximum is found in the 

 hollows. From the foregoing description it will be seen that on 

 the clay shale areas a true subsoil is found which consists of clay. 

 The mould or surface soil- is porous, especially so on timbered areas. 

 The clay subsoil, like all other clays, is not easily saturated, but 

 when once well wetted is not so readily dried. The mould acts 

 as a sponge and retains a large amount of rain water, giving the 

 water gradually down to the clay. The clayey layers and the 

 shale offer considerable obstruction to the passage of water, 

 especially the latter, although large quantities of water are absorbed 

 wherever roots have penetrated. The writer knows of the existence 

 of several wells excavated to a depth of ten or twelve feet below 

 the surface and are bottomed on to compact shale. The wells are 

 situated within a few yards of a deep railway cutting, and, although 

 unlined, retain water. The action of rainfall in augmenting the 

 quantity of water in the wells unconnected by drain pipes from 

 roofs, tfcc. has been observed. The augmentation being the entire 

 result of soakage from the surrounding ground. After continuous 

 rains the water soaked in freely and uniformly from the soil — 

 from the subsoil and partially disintegrated shale the water came 

 in small springs, a close examination of which usually revealed 

 the site of a decayed rootlet. With a suliiciency of rainfall the 

 wells became filled up with water. The rain ceasing, the level of 

 the water descended somewhat rapidly at first and then very 

 gradually, until at about four feet from the surface it became 

 stationary. The soakage water which drains into freshly made 

 excavations at a depth of from ten to twelve feet below the surface 

 is usually brackish. The depth of unlined wells is therefore 

 limited if the water is required for domestic purposes. The 

 presence of the soluble saline matter at such a shallow depth is 

 a striking proof of the lateral impermeability of the strata, more 

 especially when the immense period of time is considered during 

 which the surface has been exposed to atmospheric influences. 



In a porous strata salts would have long ago been removed. 

 The presence of natural basins oil the surface of the clay shale 

 areas is exceptional — none are known to the writer. This is a 

 favourable feature. Artificial basins formed for domestic or 

 industrial purposes are far from uncommon. 



