114 E. J. Dana: Notes Northern Territory. 



then evaporated, causing the salts in the sea water to crystallise, 

 and so gradually, grain by grain, the rock is disintegrated, leaving 

 the surface quite rough and disclosing the original sand grains, of 

 which the sandstone was constituted before the interstices between 

 these grains were filled in by secondary silica, and the rock trans- 

 formed into quartzite. 



When the rock is wetted mechanical effects are produced on its 

 texture by the crystallisation of the salts present in the sea-water. 

 Whether in addition any chemical effects are produced is not cer- 

 tain. Such might be the attacking of the secondary silica present, 

 which acts as a cementing substance, binding the grains of sand 

 together. Certain appearances suggest that this cementing sub- 

 stance has been removed. 



The sharp edges of the blocks are first attacked, and all angles 

 are removed; in time the whole mass is reduced to a rounded 

 form simulating the rounded forms due to moving water, except 

 for the rough granular surfaces. Every stage is in progress, and 

 perfectly rounded pebbles such as shown in figures 1 and 2 ulti- 

 mately result. The blocks are not moved but disintegration takes 

 place in situ. The solid quartzite at high water mark is disinteg- 

 rated in the same way as the loose blocks. 



As to the time occupied in reducing a large angular block of 

 quartzite to a well-rounded pebble no idea could be formed. The 

 quartzite is unusually hard, breaking with a clean, even fracture, 

 and under other conditions would remain unaltered indefinitely, 

 and it seems remarkable that such a resistant, material to ordinary 

 disintegrating agencies should yield so readily to salt water. 

 Whether the process is slower than where attrition by moving water 

 occurs is uncertain, but probably the alternate wetting and crystal- 

 lising effects a fairly rapid reduction in size. 



The process by which these salt-water pebbles are produced is not 

 unlike that which is universal where fresh water attacks rock 

 surfaces by penetrating cracks and fissures, perhaps originally 

 caused by expansion and contraction of the rock mass. The water 

 in these cracks and fissures becomes frozen, expanding in the 

 process, and thrusting the particles and fragments asunder. Crystal- 

 lisation is the prime cause of the energy exerted in both cases. 



