260 



the lake area are compressed into thin, compact, fresh -water 

 limestones ; whilst the bed near Orroroo is a more or less 

 open calcareous tufa. The stems, nodes, and branchlets of 

 the Cliara can be individually recognized, but they have been 

 thickened by secondary deposition of carbonate of lime. It 

 is evident that at this spot a calcifying spring drained into 

 the lake or waterhole contemporaneously with the growth of 

 the aquatic plants and has petrified the (Jhara as they stood 

 in the water. On one slab a calcified patch of confervge 

 growths can be clearly distinguished. A number of small 

 fresh-water shells occur with the (Jhara stems at this spot. 

 They have been submitted to several Australian concholo- 

 gists for' determination, and it is believed that tlley belong 

 to the genera Fotamo2}i/rgus or BythinicTIa , but as no one in 

 Australia is at present working on this group there is some 

 uncertainty as to their true position. 



This latest find is interesting as showing the extension 

 of quiet and permanent waters on the alluvial benches fac- 

 ing tlie Black Rock and Orroroo Valley, through which it 

 is suggested that the main river at one time flowed. The 

 calcareous Cltara bed occurs close to the last vestige of the 

 old rocks in Pekina Creek before they disappear under the 

 alluvial of the plain (fig. 2). These old rocks represent the 



Upper 



; Orroreo 

 ; Plain 



TAPLEY HILL SLATES (Lower Cambrian) 



— '^LUVIATILE-- -^ 



feet; - 



Fig. 2. 



Diagrammatic section of the lower part of Pekina 

 Creek and the Orroroo Plain, showing the deep alluvium filling 

 the Trunk Valley and its overlap of the shelf of old rocks border- 

 ing the valley. The lacustrine areas would form the back- 

 waters of the main river when at its maximum of aggradation. 



scarp-face of a buried valley, and the running stream of 

 Pekina Creek seldom persists more than a few yards beyond 

 their limits, which is aboiit half a mile above the railway 

 bridge that crosses the creek. Except for a day or two after 

 a great flood, the water east of that point ceases to flow at 



