OF NEW SOUTH WALES. 29 



of the upper limestone is full of lumps of shale. These con- 

 siderations may account for the absence of the larger corals, and 

 together with the fact 'that many recent species of Trochus, 

 Avicula and Geplialoijoda are found between low water and 50 

 fathoms may explain why the first remains of the fauna of the 

 seas that succeeded the sinking of the old shore — now represented 

 by the grit with its ripple marks — should be such a group as we 

 there find of Modioloijsis, Avicula, MurcMsonia, and Orthoceras. 

 That the sinking was interrupted by periods of upheaval, seems 

 probable from the thin layer of sandstone covered with AvicuUdce 

 that occurs above the strata of Spirifers and Pterinea. This thin 

 layer seems to e!xplain a rather notable thing connected with this 

 Spirifer bed. Evidently, to my mind, these Spirifers were not 

 buried in the order their fossil shells now lie. From a foot 

 below the Spirifers to this sandstone layer everything appears 

 sorted. First we have a layer of irregular lumps, then one of 

 smaller lumps, often containing a MurcMsonia, a Loxonema, and 

 an occasional Spirifer ; then a layer of almost nothing but Spirifers 

 as close together as possible, but scarcely an inch thick ; then 

 Pterinea two or three deep, but not making a thickness of half 

 an inch; then smaller AvicuUdce and sandstone. Does it not 

 seem probable that the temporary rising of the sea bed brought 

 the burial place of these mollusca within the action of the tide, 

 whose ebbing and flowing has sorted them in a manner that no 

 other agent but running water, as far as I know, ever does. 



Now AS TO THE Age. 



It cannot be expected to establish in Australian geology the 

 minute subdivisions it has been found necessary to make in the 

 geology of other distant countries ; very unlikely, for instance, 

 that the same breaks should occur here as in England and 

 America. The division of upper and lower Silurian not arising 

 from local arrangements has been made out in all countries where 

 the Silurian system is at all developed. I therefore consider the 

 terms upper and lower Silurian can be used here. The Yass 

 Period, or the period of the deposition of the Yass and Hume 



