BY PKOFESSOR STEPHENS, M.A. 553 



doubtedly what we call Devonian. Upon the summit of this 

 limestone and filling its crevices, we find a kind of secondary 

 deposit of travertine mixed with a greater or less proportion of 

 silicious matter. It frequently includes fragments of the orio-inal 

 limestone and of slaty rock which appears to have been the 

 same as the associated slates, but in a less metamorphic condition. 

 I have observed precisely the same phenomenon on limestone 

 summits near Tam worth, Moara Creek, and Attunga. It is quite 

 different from the formation of travertine in a river bed, as 

 at AYallerawang. (See p. 40Ji). I do not doubt that this flinty 

 travertine is a deposit from springs which at one time rose to the 

 surface through joints, fissures or faults of the limestone, bein^ 

 derived from the internal drainage of high silicious rocks in the 

 vicinity. The frequent silicification of corals and other organic 

 remains in these Devonian limestones, together with the more or 

 less complete abstraction of carbonate of lime from the structure 

 of the same fossils, even when not replaced by silica, seem to 

 indicate that the water was warm, and charged moderately both 

 with silica and carbonic acid. It would under such conditions 

 be likely to dissolve the limestone with which it came in contact, 

 and at the same time to deposit both silica, as it cooled, and lime, 

 when exposed to the air. Such waters might be expected to soak 

 down through the silicious and permeable rocks of this formation, 

 which at the time formed mountain masses of great height and 

 extent, and to rise again to the surface where they found an 

 outlet. The outlet in this case can hardly have been submarine, 

 as we cannot suppose that the lime held in solution would be 

 deposited at the bottom of the sea, where the exce^^s of carbonic 

 acid would keep it dissolved. We must therefore suppose this 

 travertine to have been formed on land, or in shallow fresh water. 

 In either case we are driven to the conclusion that the existing 

 ridge was at one time a valley, and that the present gullies on 

 either side were its containing ranges. 



