F. Chapman — On Concretionary Nodules. 553 



Y. — On Concretionary Nodules with Plant-Remains found 

 IN THE Old Bed of the Yarra at S. Melbourne ; and 

 their Resemblance to the Calcareous Nodules known as 

 ' Coal-Balls.' 



By -Frederick Chapman, A.L.S., etc., 

 Palaeontologist to the National Museum, Melbourne. 



ALTHOUGH many of the nodular bodies met with in sedimentary- 

 rocks which were formerly held to be concretions or simple 

 aggregations of mineral matter have since been found to be due to 

 the work of minute animals or plants, there is yet a more numerous 

 class of true concretions which were undoubtedly formed by 

 chemical reaction in the surrounding water and sediments ; the 

 resulting precipitation being often deposited upon nuclei of organic 

 origin, as fish, woody fragments, or shells. Occasionally these 

 nodules were of subsequent formation to the deposit in which they 

 occur. In nodules which include organic remains, the segregation 

 was accelerated by the partial decay of the included organic 

 fragments. 



To the latter class belong some interesting examples of spheroidal 

 concretions lately brought under my notice by a keen observer, 

 Mr. F. Spry, of Melbourne. In June, 1900, sewerage excavations 

 were being carried on in S. Melbourne, and at Power Street, near 

 Grant Street, 16 feet below the surface, the concretionary nodules 

 were found which form the subject of this paper. The workmen 

 here struck the bed of one of the old channels of the Yarra estuary. 

 At the time of the formation of this deposit (probably late 

 Pleistocene) the mouth of the Yarra was situated three or more 

 miles to the eastward of the present river mouth, the Saltwater and 

 Yarra Rivers having had separate outlets, instead of, as now, the 

 former joining the latter before entering Hobson's Bay. The 

 earlier course of the Yarra to the sea was thus shortened by 

 about six miles. The estuary was evidently much farther back in' 

 late Pleistocene times; but since then the shore-line has been 

 gradually encroaching on Hobson's Bay, owing to the vast 

 quantities of silt carried down by the Yarra and Saltwater Rivers, 

 and also by the drifting of the shore-sand from the eastward, which 

 has been continuously supplied by the disintegration of the Tertiary 

 cliffs from Brighton to Beaumaris. This process of filling up has 

 more than kept pace with the subsidence of the area of the Yarra 

 Delta, as indicated by the level of the old river channel, which 

 is now found at five feet below low-water at S. Melbourne. Since 

 the actual subsidence of this area, however, there has been what 

 is perhaps best termed a negative movement of the shore-line, 

 resulting in a six-foot raised beach, preserved at various parts around 

 Port Phillip Bay. 



The details as to the conditions under which the clay nodules 

 were found have been kindly supplied me by Mr. Spry, who 

 considers that the channel, to which reference has been made, was 



