28 THE PROCEEDINGS OF THE LINNEAX SOCIETY 



In contrasting these beds lifchologically, the large proportion of 

 grit dividing the Yass beds into such unequal halves, and the 

 absence of those large limestone masses which form so prominent 

 a feature in the Hume beds, are the first things to attract atten- 

 tion. Then we have in the Hume beds an almost entire absence 

 of that somewhat symmetrically transverse jointed structure ex- 

 hibited in most of the Yass grit and limestone, and instead, a 

 preponderance of concretionary structure in limestone and shale. 



Then as to the fossils — the absence in the Yass beds of large 

 corals generally, and especially of those masses which are so 

 remarkable in the Hume beds is also to be observed. The fossils 

 of the Hume beds are further distinguished by the number of 

 individuals, large size and variety of species and genera of 

 Trilobites, the number and variety of Pentameri, and by the 

 presence especially of Pentamerus ohlongus ; by the number and 

 variety of the genus Orthisj and by two layers of limestone 

 containing Fucoids ; also by their being distributed with an 

 approach to uniformity through a great thickness of strata, 

 whereas in the Yass beds all the fossils are collected in bands 

 which, if taken altogether, would not be more than from 10 to 

 20 feet thick. 



The character imparted to fossils of the Yass beds is due to the 

 great number and variety of plaited Spirifers, and the number 

 of Murcliisonia, Bellerophon and Pterinea. Peculiar to these beds 

 is also a Maclurea, a large Helix-like Troclius, an EcculioniphaluSf 

 and a small Trilobite, with a nearly globular glabella covered 

 with tubercles. 



These differences of the fossils of the two series of beds cannot 

 however, as I think, be considered as altogether marking the 

 characteristics of the different geological periods. The local 

 conditions under which these deposits have been formed have 

 undoubtedly done much to produce the results we now see. When 

 the Yass beds were being laid down, there must have been at 

 least four principal changes of level, two of elevation and two of 

 depression, as evidenced by the double series of bands of grit 

 beds, separated by intervening shale. The waters were tolerably 

 troubled too during the deposition of some of these beds, for some 



