406 ON THE GEOLOGY OF YASS PLAINS, ( THIRD PAPER). 



The inclusively Silurian Genera referred to above are : 

 Cheirurus, Calymene. 



SpJierenocJi us . Acidasp is . 



jEncrinurus. 



Professor Koninck's determination of some of the New South 

 Wales Fossils tends to establish these beds to be Upper Silurian. 

 Several fossils declared by him to be Silurian, are found in the 

 Yass and Hume beds. While those which he has described as 

 Devonian and referred to the neighbourhood of Yass, are only to 

 be found in the Murrumbidgee beds, which occur some 8 or 9 

 miles from Yass, overlying uncomf ormably the Yass beds. 



Occurring in the midst of these Silurian fossils is found what I 

 believe to be undoubtedly a Calceola and also Atrypa desquamata. 

 Now both these have only been found in other Countries in 

 Devonian strata. 



When Banaude found Goniatites, hitherto considered Devonian, 

 in the midst of his Silurian types, he suggested in explanation, 

 his theory of Colonies. 



This explanation was approved of by Sir E. Murchison, and 

 considered by Professor Forbes to be highly philosophical. The 

 presence here of Devonian forms in the midst of typical Silurian 

 species may probably be similarly explained. 



At any rate it has been specially laid down by Lyell, and is I 

 believe an universally recognised principle, that it is in the 

 highest organized forms that we must seek the type whereby to 

 characterize the age. 



I think therefore the evidence derived from the Trilohites must 

 be considered decisive, notwithstanding the presence of these two 

 lower organized forms, usually regarded as Devonian. 



The conditions under which the Yass and Hume beds were 

 deposited, were evidently similar to those which existed during 

 the formation of the Silurian of other countries. There is no 

 warranty for assuming that the deposits were formed in oceanic 



