212 Influence of the Physical Character 
neighbourhood, and on the surface, as caves or great hollows 
are rare in the crystalline rocks, and when they do occur, 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 gulley 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 ponds 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 water. 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 gulley 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 
