68 Proceedings of the Royal Society of Victoria. 



other rock of Silurian aspect. The existence of this river on the 

 ridge between the deep valleys of the Pyrete and Goodman's 

 Creeks testifies to the amount of denudation which nas taken 

 place here in post-basalt times. The sides of the pre-existing 

 valley of the river, which guided the basalt How, have entirely 

 disappeared at a point quite 500 feet above the creeks on either 

 side of the ridge. 



With respect to the physical circumstances under which the 

 Coimaidai series was formed, it is clear that these beds accumulated 

 under fresh water conditions, fluviatile and lacustrine. A large 

 depressed area existed in the Bacchus Marsh district, in which 

 was a lake resting on Silurian and Glacial beds. The shore line 

 of the lake was doubtless irregular, and one large indentation 

 extended up to the Coimaidai district. The deepest part of the 

 lake was towards Bacchus Marsh, but in the region of the present 

 valley of Coimaidai was a basin of considerable depth. Into 

 this lake the ancient river previously referred to flowed in a 

 southerly direction, accumulating, probably with the help of 

 other streams, the masses of gravel and drift we now see. The 

 main current of the river seems to have been towards the west 

 side of the plain, although heavy boulder drift on the west 

 of the Pyrete indicates that either a channel of the river came that 

 way or marks the course of other creeks emptying into the lake. 

 A spur of Silurian, probably forming a bold promontory, seems 

 to have sheltered the Coimaidai Bay from the main flow of the 

 river, and in this bay special conditions appear to have obtained. 

 The regular stratification and fine lamination of the limestones 

 imply fairly still water continuing during a considerable interval of 

 time. Certain pit-marks, which may have been caused by rain, in 

 the limestone suggest that there may have been variations of the 

 level of the lake, by which expanses of the bed may have been 

 exposed. 



The limestone is a dolomitic travertin, and undoubtedly of 

 chemical origin. Lime and magnesia, in solution in the waters 

 of the lake, have been precipitated as carbonates, probably 

 by springs of COo. In this context it may be mentioned that 

 springs of CO., are at the present day abundant along the course 

 of the Pyrete Creek, and in every case in which we have noticed 

 them the bed-rock from which they issue has been either Glacial 



