33 



bed of sand and gravel was met with which varied in 

 thickness from 4 ft. to 9 ft. On looking over the gravel I 

 discovered a boulder, about 5 in. in length, which had an 

 open vesicular structure, very similar to scoriaceous lava. 

 Its presence under such circumstances seemed unaccountable, 

 so the specimen was laid aside with the hope that at some 

 future time some evidence might be obtained that would 

 throw light on its true nature and origin. 



My interest in the subject was revived when, in 1914, 

 Mr. F. R. Zietz placed in my hands a similar water-worn 

 stone which he and Mr. Ridgway had obtained from the 

 extensive gravel deposits at Findon, adjacent to the Grange 

 Road, about four and a half miles from Adelaide. Sub- 

 sequently, Mr. Ridgway, who takes much interest in these 

 gravels and their incidental contents, was successful in 

 obtaining several additional examples of a like kind, the 

 largest of which is a subglobular mass, rounded irregularly 

 by river attrition, and measures 32 in. round the longer 

 diameter and 25 in. round the lesser. In each instance the 

 contemporaneity of these vesicular stones with the rest of 

 the pebbles in the bed was evidenced by portions of the 

 adjacent sand and small stones having become cemented to 

 the exterior surface. 



The first impression was that these scoriaceous-looking 

 stones were of volcanic origin, as in their general appearance 

 they can scarcely be distinguished from such vesicular lava 

 as occurs in volcanoes of the Mount Gambier and Mount 

 Schank type. The latter localities are, however, too distant 

 to give any probability of these stones having come from 

 such a source. It seemed equally improbable that extrusions 

 of lava existed in the hill regions that formed the hydro- 

 graphic basin of the River Torrens. 



In pursuance of these enquiries it then became necessary 

 to make thin sections for examination by the microscope, and 

 also to submit the stones to chemical analysis. The microscope 

 showed that fusion had taken place and that the vesicular 

 structure arose from this cause, but whilst mineral glass was 

 present, there was an absence of any further evidence of 

 volcanic action of an incidental kind. There could also be 

 detected in the transparent section minute grains of quartz 

 (sand) which had not undergone fusion. Further, when the 

 largest fragment obtained is examined by the naked sight, 

 small pockets can be noted in the mass which are quite 

 distinct from the gas vesicles, and some of these contain 

 earthy matter which must be regarded as portions of the 

 original material which had not undergone fusion. 



