93 



two opposite walls eighteen inches thick in a house of an ac- 

 quaintance opens out wide every summer and contracts closely 

 every winter. Some large cracks had been observed in the 

 ground before building but were regarded as superficial 

 merely. Whence do these fissures lead ? Direct observations 

 prove that these adhesive clays are succeeded by layers of 

 limestone concretions, gravel beds, decaying calcareous slates, 

 &c. As all these are very porous, they afford easy channels 

 for the drainage of the surface waters and the materials 

 carried in a comminuted state by the same, and thus surface 

 watercourses are rendered unnecessary. If this process be 

 frequently repeated, as is no doubt the case every year, it 

 follows that the friable, fertile soil around the vents must 

 gradually diminish, and shallow depressions be formed in the 

 course of time, slower or faster, as local circumstances favour. 

 Only one instance remarkable for rapidity of effect can be 

 cited. In a " Bay of Biscay soil " region, on a somewhat steep 

 slope descending from New Mecklenburg plain towards the 

 River North Para, west of Tanunda, a hole was noticed in the 

 ground after a heavy rain ■ its diameter was about three 

 inches, the depth rather more, the sides steep. A small rill 

 of flood-water escaped into it, without overflowing in the 

 direction of the steep banks of a brooklet some 20 or 30 yards 

 distant. This insignificant vent gradually widened, and in 

 less than two years had augmented to three feet in depth, and 

 over four feet diameter. 



The conclusions deduced from the foregoing are : — (1) If 

 the subterranean vents be relatively few, the depressions 

 will be correspondingly apart ; (2) the longer the time has 

 been during which operations as described have been carried 

 on, the narrower will be the ridges between the depressions ; 



(3) the more friable the soil the steeper will be the slopes ; 



(4) if the fissures be very small, numerous, and equally 

 distributed, assuming uniform porosity of the subsoil also, 

 none or very shallow depressions will be formed, because 

 every part of the area will be reduced simultaneously at the 

 same rate nearly. Thus the depth and distribution of the 

 depressions depend on inequality of distribution and size of 

 the temporary fissures, the friability of the soil, the porosity 

 of the subsoil, and time. 



If the nature of the soil is such as to admit of very rapid 

 expansion, and consequent long detention of the muddy 

 waters, a fine sediment will collect in the bottom, which, 

 drying hard when exposed, will at last prevent any deep 

 fissures from forming, and thus constitute what in colonial 

 parlance are termed " claypans " — a blessing for all regions 

 destitute of permanent springs. Some of these are scattered in 



