Tlie Geology of Coimaidai. 67 



werribeetisis. This band is at the lowest point in our series, the 

 first appearance of limestone being about 130 feet above it. 

 The relation of the Coimaidai series to the well-known Bacchus 

 Marsh leaf beds is a point of great importance, especially as 

 Messrs. Hall and Pritchard and Mr. C. Brittlebank have 

 advanced strong evidence that the latter beds are at least of 

 Eocene, if not of Cretaceous, age. Between the sands, gravels 

 and ferruginous beds of varying character forming the Bacchus 

 Marsh leaf beds and those of the Coimaidai area there is a very 

 strong lithological resemblance. The absence of leaves from the 

 Coimaidai series is, however, so marked a feature that, in our 

 present knowledge of the facts, we are inclined to regard the 

 Coimaidai series as lying above the Bacchus Marsh leaf beds. 

 Except for the non-discovery of leaf-impressions, we have no 

 evidence to show that there is a break or discontinuity between 

 the Coimaidai and Bacchus Marsh series. On the contrary, our 

 sections would seem to show that there is a gradual passage from 

 the pipe-clay, in which we found our only leaf-casts, and 

 which we regard as on the horizon of the Bacchus Marsh leaf 

 l)eds, into the overlying series. In the Quarter-sheet of the 

 Geological Survey, Section 12 N.E. Bacchus Marsh, a small part 

 of the district we have been investigating is included, and here 

 the infra-basaltic beds are mapped as identical with the leaf beds, 

 a circumstance which corroborates the view we have taken of the 

 mutual relations of the two series. 



Before proceeding to discuss the physical condition which 

 obtained during the formation of the Coimaidai beds, attention 

 must be drawn to the existence of an ancient river- bed which has 

 played an important part in determining the present contour and 

 aspect of the country. The course of this river is, in the 

 northern part of our district, mainly coincident in position with 

 the tongue of basalt which separates the Pyrete and Goodman's 

 Valleys. As it is a gold-bearing lead, many tunnels have been 

 driven into it, and, both from the surface outcrops and the drifts 

 exposed in the tunnels, we surmise that the river was of some 

 magnitude. Some of the boulders of quartz seen by us are 

 several feet in diameter, and well rounded and waterworn, as 

 though they had travelled a considerable distance. The river 

 course is through Silurian strata, and the river-drift is quartz and 



5a 



