FhysiograiJltii of Yarra, etc. 511 



at right angles to the direction of flow of the stream at the 

 particular part in question, and the dip is westerly at 60 deg. 

 Numerous pot-holes occur for a distance of about 30 feet along 

 the stream-bed. Simple holes are generally roughly circular 

 in outline, and vary from about three inches to over a foot in 

 diameter (this measurement often being directly proportional 

 to the width of the shaly beds), with an average depth of about 

 six or eight inches. In places the holes have coalesced to form 

 compound ones, the largest of which is about four feet in 

 longer and two feet in shorter diameter, with a depth of about 

 two feet. The holes are mostly confined to the shales even 

 where two thin sandstone bands are within a few inches of one 

 another. The softer rocks (the shales) have been selected, and 

 the sandstones have generally resisted enci'oachment, with the 

 result that the enlargement and union of holes have taken 

 place jiarallel to the sandstone bands. The compound holes 

 are often therefore narrow and elongated, and together with 

 the simple ones are generally arranged in rows parallel to the 

 strike of the rocks. In some instances the sandstones have 

 been pierced both by simple and compound holes, but where 

 the diameters are uneqiuil, the longer one follows the line of 

 least resistance, which is in the direction of the strike of the 

 rocks. 



There is another aspect of some interest. The height of the 

 sandstones on the floor of the stream determines its general 

 level of erosion, and hence the deepening of pot-holes in the 

 shales does not directly lower the general surface of the 

 stream-bed. Indirectly however, the erosion of the shales has- 

 tens that of the sandstones, and consequently the floor of the 

 stream as a whole in the following manner. At low water, the 

 sandstones project as bars above the water, which occupies the 

 hollows in the shales. The sandstones thus become greater 

 targets for the atmospheric agents of frost, rain and changes 

 of temperature, than if they formed one nearly dead level. 

 In the same way at high water and flood time, the sand and 

 gravel carried along the stream bed, have a greater surface of 

 sandstone to abrade than if the floor of the creek were level, 

 notwithstanding that the hollows become partly filled with 

 gravel, sand and other debris. 



