SILURIAN AND DEVONIAN ROCKS. 139 
Braidwood beds, both of which are generally considered, 
mainly on the presence of Lepidodendron australe, to be of 
Upper Devonian age. If we take the Canobolas beds as 
being of this age, it becomes necessary to account for the 
absence of the middle and lower Devonian strata (this 
applies also to the Mount Lambie and Braidwood districts). 
There are two possible explanations: (1) the Lower and 
Middle Devonian strata had been entirely removed by 
denudation prior to the deposition of the Upper Devonian, 
or (2) the areas where the Upper Devonian now occur 
were dry land during the Lower and Middle Devonian 
epochs. Both suppositions, but particularly the former, 
demand a long interval of time between the close of the 
Silurian and the beginning of the Upper Devonian, during 
which there must have been profound denudation and 
corresponding deposition elsewhere. Much might be said 
both for and against these two explanations, but in view 
of our present scanty knowledge of the Devonian strata of 
New South Wales, it is perhaps premature to endeavour 
to arrive at any definite conclusions. There is one other 
possible explanation that appears worthy of consideration. 
So far as the writer knows, wherever the so-called Upper 
Devonian strata occurs in New South Wales there is an 
absence of Middle Devonian strata, and conversely, where 
so-called Middle Devonian strata occur, such as the Mur- 
rumbidgee beds, there appears to be an absence of Upper 
Devonian beds. Might not these two types of Devonian 
strata be contemporaneous, the lithological and paleeonto- 
logical difference being due to the different conditions under 
which each was deposited; the Murrumbidgee beds being 
deposited in a more or less open sea, far removed from a 
shore line and containing an abundant coral fauna, while 
the beds of the Canoblas, Mount Lambie type, were deposited 
ina Shallow coastal sea, which was receiving abundant and 
relatively coarse sediment from the adjacent land. 
