302 Dr. Malcolm Maclaren — Deseii-nrder in TV. Australia. 



Gnamma holes are most common in granite areas, but are occasionally 

 to be found in the quartz-porpliyry and soda-porphyry dykes tliat 

 seam the greenstone-schists. Somewhat similar rock-holes may occur 

 in the lateritic deposits that mask tlie greenstone-schists, but they are 

 there very rare. The normal shape is usually that of an ordinary 

 water carafe, having a comparatively narrow cjdindrical orijfice and 

 widening out greatly below (Fig. 1). Their capacity varies fro7u 

 a few gallons to several hundreds and even thousands of gallons, 

 and several may be found on the one rock slope. Like the soaks 

 they are dependent on the rainfall of the previous winter. Under 

 favourable circumstances they may obtain sufficient Avater to last 

 throughout the summer, but they cannot, in individual cases, 

 always be relied upon. The scanty rains of the eastern desert are 

 erratic in distribiition, falling always as a few heavy local showers, 

 so that a given rock slope may escape the rains of a whole winter. 

 The water of these gnammu holes, forming as it does the principal 

 summer supply of the wandering aboriginals, is carefully preserved 

 by them both from evaporation and from marsupials, reptiles, and 

 birds, by covering the water with eucalypt foliage, and the orifice 

 with flat stones. 



The occasional occurrence of these peculiar rock-holes at the inter- 

 section of or along joints in massive granite gives some clue to their 

 origin. They apparently arise in the first place from the flaking off 

 by insolation of small fragments along the walls of the joint. In 

 the cavity thus formed (generally on a smooth sand-swept rock 

 surface) a little water lodges, in time decomposing the felspathic 

 member of the rock, and freeing the quartz-grains which are swept 

 from the shallow depression by the high winds of the desert. Seasonal 

 repetition of this process slowly deepens the hole beyond the I'each 

 of the wind. The sand now retains the water, and the sides and 

 the bottom of the cavity are alone attacked, and the hole grows in 

 size horizontally and vertically. The orifice is protected from 

 enlargement partly because there is no standing water to act on its 

 sides, but much more because of the siliceous cementation that has 

 taken place at and near the rock surface in its neighbourhood, as 

 a result of the upward capillary movement of the silica dissolved in 

 the course of the decomposition of the felspars of the granite. This 

 superficial cementation is characteristic of this desert region, and 

 has its utmost expression, as 1 propose to show in another place, in 

 the formation of beds of siliceous ' laterite ' 30 and 40 feet thick, 

 comparable in respect of origin with the ferruginous laterites of the 

 greenstone-schist areas. To it must be ascribed the unique form 

 of these granite rock-holes. The process of internal enlargement 

 has probably been greatly accelerated by the removal of the sand 

 from the bottom by human agency, since the position of all these 

 rock-holes is well known to the aboriginals, who, indeed, govern their 

 migrations by the location and capacity of the various gnammas. 



In the camp-fire conversation of the West Australian prospectors and 

 cattlemen, the all-engrossing subject is water, and, when on this topic, 

 it is seldom long before the ' night wells ' are mentioned (Fig. 2). 

 These are described as rock-holes dry by day, but furnishing an 



