Geological Notes Northern Trri'itor;/. 1 L3 



filled burrow's were sometimes nearly vertical, while the normal 

 bedding was horizontal. The section cuts these sand filled burrows 

 at all angles, hence the strange and curious markings. 



Saltwater Pebbles. 



During a visit in June, 1913, to the Victoria River, Northern 

 Territory, an interesting and to me novel method of pebble forma- 

 tion came under notice. The Victoria River rises in the Barkly 

 Highlands, and in the lower part of its course flows through a 

 sunken valley, which has been tilled to its present level by fine silt 

 brought down by flood waters from the higher country drained by 

 its sources. At the river's mouth a great delta is being formed of 

 similar material, which extends as islands for miles out to sea. 

 Most of this deltaic material is just covered at spring tides, and is 

 being consolidated by the agency, of two or three species of man- 

 groves. 



Where the river debouches into the sea, silt constitutes about one 

 twenty-fifth in bulk of the muddy water. The mangroves perform 

 the initial work of settling this sediment and converting it into 

 solid land. 



Along the main valley and in the branch valleys the silt has 

 accumulated to a level only covered by high tides, and forming 

 narrow and in places wide mud flats. 



When the.*e impinge upon the hills high-water mark is strewn 

 with blocks and fragments of rock that have rolled down the gene- 

 rally steep slopes on to the edge of the mud flats. The hills are 

 formed of shales, with bands of extremely hard quartzite of various 

 shades of colour, that range from a few feet to over 100 feet in 

 thickness. Inland these quartzite bands form precipices that make 

 very secure boundaries quite inaccessible for miles on end. Later 

 on, when the country becomes occupied, these may be utilised as 

 portions of enclosures. 



As the rocks crumble down the shales quickly become disin- 

 tegrated, and the quartzite blocks are principally represented along 

 the high-water line. At first the blocks and fragments of quartzite 

 are angular, as they reach the edge of the mud flats. At high 

 tides the salt water reaches these blocks as they lie in the mud and 

 the wind causes the spray to cover them with salt water. When 

 the tide recedes an incrustation of saline material coats the surface 

 of the quartzite. Alternately the quartzite blocks are wetted with 

 salt water and dried by a tropical sun, with the result that the 

 water enters every minute crack and fissure in the surface, and is 



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