186 



structures. 



Eocks. 



JReptiles. — Extinct reptiles, known as Lahijrintliodonts by 

 reason o£ the intricate structure of tlie teeth, are occasionally 

 found. Specimens of these fossils will be found in the Strati- 

 graphical Series. The fossil evidence shows that these strata 

 were laid down on the estuary of a large river. 



Columnar Sandstone is the most interesting structural peculi- 

 arity to be seen round Sydney. It has been noticed at quite a 

 number of localities, of which Bondi is the best known. The Bondi 

 quarry has now been partially reserved, but much of it has been 

 destroyed. Columns will also be found in cases 131 and 138. 

 Sometimes they are long and slender, at others short and thick ; 

 most commonly they are five-sided. The cause of this structure 

 is not yet understood. At Bondi, igneous dykes are associated 

 with it, and these are generally stated to have been the direct 

 cause ; nevertheless, elsewhere, for instance, near Pymble, no 

 igneous rock can be found. The true explanation has yet to be 

 found. A similar occurrence is known in the Inner Hebrides, 

 Scotland. 



Photographs of Columnar Sandstone at Bondi and Lane Cove 

 Eiver (Pymble) are on view. 



Hipjple-marlcs. — That the Hawkesbury Series were laid down 



in shallow water is evident from the abundance of ripple-marked 

 slabs of shale and sandstone. A large slab of such sandstone 

 stands near the door, and a similar piece of shale from the 

 Balmain Pit hangs over these cases. 



False Bedding. — This is another evidence of shallow-water 

 conditions. It cannot well be illustrated by museum specimens, 

 but photographs are on view. 



Aluminous Ironstone, Quarantine Ground. 

 Basalt. — Many dykes. 



Breccia (shale shattered by volcanic action). 

 Conglomerate. 



Altered Bolerite (diabase). — Prospect, IS'arrabeeu. 

 Gahhro. — Boulders included in the basalt at Dundas ; also a 

 dyke at the Basin, Nepean Eiver. 



