25 



Geological memoranda (Second Contribution). 



By Professor Walter Howchin, F.G.S. 



Subjects : 

 I. Miniature Serpuline " Atolls. " 

 II. Pseudo-Cryptozoon Structure. 



III. A Prehistoric Alluvial Fan of Exceptional Character 



at the Mouth of the Glen Osmond Gorge. 



IV. The Occurrence of Scoriaceous Boulders in the Ancient 



Gravels of the River Torrens. 



[Read May 12. 1921.] 



Plates V. to VII. 



I. Miniature Serpuline "Atolls." 

 PL v. 



Professor L. V. Pirsson, in the Text Book of Geology by 

 Pirsson and Schuchert, has figured and briefly described 

 [loc. tit., Part I., p. 180, fig. 152] some small atoll-like 

 formations that occur on the coasts of the Bermudas. The 

 explanation given of the figure is as follows: — "Serpuline 

 Atolls, Bermudas Islands. These structures, formed in 

 shallow waters, may be a, number of feet or yards in diameter 

 and are locally called 'boilers.' " 



Interesting examples of a similar kind occur on the 

 coast, at Encounter Bay, South Australia. The littoral zone, 

 at the locality mentioned, is very shallow and takes the form 

 of a marine platform which extends a considerable distance 

 seawards. This submerged shelf has been cut out by the 

 waves in the Permo-carboniferous glacial till, while large 

 erratics, washed out from the latter, are strewn along the 

 beach and occur abundantly in shallow water of the sea 

 margin. W The bay is largely land-locked, being sheltered 

 by the outlying islets of Granite Island, Seal Rock, and 

 Wright Island, as well as by the promonteries of Victor 

 Harbour, at the north-east angle, arid Rosetta Head, at the 

 south-west. 



(l) Howchin, "The Glacial (Permo-Carb.) Moraines of Rosetta 

 Head and King Point," Roy. Soc. S. Anstr., vol. xxxiv. (1910), dIs. 

 vii. and viii. 



