134 Reviews — Northern Territory Geology, Australia. 



reed, Phragmites communis, Trin., /3. flavescens, Gren & Godr. These, 

 with the soil which they retain by the aid of their roots, form 

 a subaqueous mass about 6 feet in thickness. This can be divided 

 into three layers : the uppermost, about 6 inches thick, with a very 

 highly organic black soil in which are rooted various water plants ; 

 the middle, about 18 inches thick, with a lighter soil containing 

 40 per cent of organic matter; and the lowest, about 4 feet thick, 

 with a soil having an organic content of only 17 per cent. Prom 

 these rhizomes grow reed shoots as high as 17 feet, which form the 

 aerial portion of the Plav. 



The formation of Plav takes the following course : first round the 

 sandbanks an " open reed swamp" forms (i.e. a swamp in which the 

 reeds are fairly widely separated from one another). This in course 

 of time by the spreading of the reeds becomes thicker, forming a 

 "closed reed swamp". This swamp then begins to lose its hold on 

 the ooze in which it is rooted by the death of some of the lower 

 rhizomes ; if then the water is not shallow enough for the mass of 

 rhizomes to fill it entirely to the surface, the hydrostatic pressure on 

 the upper layers will cause the mass to rise, and it will become 

 floating reed swamp or Plav. The three necessary conditions then 

 for the formation of Plav are : that the water should be sufficiently 

 deep ; that the floods do not bring down much silt which would 

 lessen the depth of the water so that the lower rhizomes could become 

 re-rooted; and that the basal decomposition should set in in the 

 swamp stage and not in the fen stage, as happens in Norfolk, where 

 the reeds fill up the whole water space before basal decomposition 

 sets in. 



Some Plavs are miles long, and are attached at one side or end ; 

 others, however, are quite small and form floating islands. These 

 latter may be broken pieces of the larger Plavs, but more probably 

 originated on a small bank so that they became free floating when 

 detached at the base. To what extent Plav may be regarded as 

 another substance for coal is not very clear. The large amount of 

 inorganic soil it contains would indicate a higher ash content than is 

 usual in most coals. If, however, the delta of the Danube were 

 invaded at some early date by the sea something very much resembling 

 coal would, no doubt, be found there. 



W. H. W. 



V. — Geological Notes, North krn Territory, Australia. By E. J. 

 Dunn. Proc. B,oy. Soc. Victoria, vol. xxviii(N.s.),pt. i, pp. 112-14. 



IN this publication Mr. E. J. Dunn describes a portion of the core 

 of a borehole, where the appearance of highly contorted strata 

 has been produced in rocks of Carboniferous age by the boring of 

 worms. The worms made tortuous holes in black mud at a time 

 when white sand was being deposited. This washed into the holes 

 and filled them up, giving the appearance of a high degree of 

 contortion. 



He also describes how the formation of rounded pebbles may take 

 place without the wearing action of water. At the mouth of the 



