212 



Influence of the Physical Character 



neighbourliood, and on the surface^ as caves or great hollows 

 are rare in the crystalline rocks, and when they do occnr, are 

 of small dimensions. 



The strata which absorb the largest per centage of storm 

 waters, in proportion to their extent, are clearly those beds 

 of sand and quartz gravel which form the beds of the creeks. 

 Perhaps the most satisfactory evidence of this is to be found 

 in the gold fields, where there are comparatively extensive 

 valleys of quartz gravel, sand, and clay. In winter these form 

 the beds of deep and rapid streams, and it is not unusual at that 

 season to walk across a deep dry guUey one day, and the 

 next to find a current of water some yards in width. The 

 water ceases to flow however very soon after the rain has 

 passed, and in some places, the labors of the gold miner, even 

 in winter, are occasionally obstructed by the absence of a 

 sufficient current. In summer the course of these streams is 

 marked by a succession of small ponds and sedgy swamps ; and 

 it is worthy of remark that the ponds are not strictly in the 

 alluvium, but appear where the tilted edges of the slate form 

 a barrier, transverse to the valley, as shown in section No.. 2. 

 A question arises here of some importance to the present 

 inquiry, — what can be offered in explanation of the seeming 

 anomoly, the impermeability of the titled clay-slate rocks ? 

 All our experience in this colony shows that there are natural 

 basins in these rocks, where very little of the contained water 

 is absorbed. I think the lithological character of these strata 

 is sufficient to explain the fact. These poqds are found where 

 the beds are composed of fine clay and mud, and are so little 

 altered, that they weather into a plastic mass ; and when this 

 fills the interstices and joints of the rocks, or covers the extent 

 of the basin to the depth of three or four, or several feet, it will 

 effectually prevent the percolation of w^ater. This of course 

 can only take place where the clay and mud is deposited slowly, 

 and cannot be held to render these rocks impermeable where 

 they rise in steep ridges. 



In sinking a shaft through the tertiary beds shown in 

 section No. 2, water is invariably found within a few feet of 

 the subordinate rock. We have abundant proof, however, that 

 the supply is limited, and that strictly according to the extent 

 of the basin, which is sufficient to show that very little water 

 is derived from the neighbouring ridges. If we examine a 

 number of shafts in one of these valleys, commencing at the 

 top of a guUey and going gradually downwards, we observe 

 that the "wet sinking" is always towards the outlet of the 

 basin ; and though the surface, even there, may be dry, and 



