Volcanic Tuff of Pfjark Marsh. 3 



result of the decay, and removal of vegetable remains. Higher 

 up in the bed, where there is no sign of those remains, the smaller 

 .and more regular cavities occur in horizontal zones, and these 

 likely enough have arisen through air entangled in the showers 

 -of ashes being carried with them to the bottom of the swamp, and 

 there unable to escape. 



In neither kind of cavity is there the slightest indication that it 

 was formed by the gases given off by the decomposition of vegeta- 

 tion in the swamp in which the ashes fell. The presence of cavities 

 due to air or other gases entangled in the shower of ashes, if it lie 

 -a correct interpretation of the phenonienon, in itself sets at rest 

 any doubts about the tuff not being in its original site, because 

 such cavities could not have been retained during a re-distribution 

 either by wind or water. 



Mahony and Grayson, 7 in describing the microscopic characters 

 of the Hampden tuffs, mention that a deposit at Blind Ci'eek, 

 about which some uncertainty exists as to whether it has been 

 re-deposited or not, is similar to the tuffs occuring elsewhere in 

 their original position. The same may be said of the Pejark tuff, 

 as its character does not seem to differ from that of tho bods of fine 

 texture undoubtedly in situ. Tlie Pejark bed is also sharply 

 defined from both the underlying and overlying deposits, and there 

 has been no mingling with extraneous material in any part of it. 

 There are, then, no features in connection with the occurrence 

 which might be held to indicate that it was laid down under con- 

 ditions inconsistent with primary deposition. 



The facts as they are, point not only to a sudden change in the 

 nature of the deposited material, but to great rai^idity of deposi- 

 tion with slight intervals of rest, during which the fine sediment 

 suspended in the water of the lagoon or swamp settled, forming 

 thin, impalpable seams in the tuff. 



The Avhole bed was no doubt laid down in a comparatively sliort 

 time, then earlier conditions again prevailed, and the present 

 alluvial soil was deposited. 



Regarding the origin of the tuff in question, there are three 

 possible sources within the immediate neighbourhood of tho place 

 where the excavations were made, viz., Mt. Noorat, three miles 

 to the north, Lake Keilambete, about two miles to the west, and 

 Lake Terang, one mile due south. 



The first of these sources and the most distant from the position 

 mentioned, while leaving no question of its crater origin, has not 



7. Loc. supra cit., p. 19. 



2a 



