GEOLOGY OF YASS DISTRICT. Ill 



of fossiliferous tuff^ is also found there, which yielded the following 

 Silurian forms : — Cyathophyllum, Favosites, Hcliolites, crinoid stems, 

 Atrypa reticularis, {?)Athyris, Spirifera, Cafnaroicechia,{?) Rhyn- 

 chotreta,{?) and Encrinurus punctatus : Brunn. sp. 



About two miles to the S.W. of Oak Range, in the bed of 

 Derrengullen Creek an outcrop of hmestones, shales, and sand- 

 stones is exposed, the series forming a continuation of the Yass Beds 

 of Jenkins. The intervening country, however, is composed mainly 

 of the No. 2 porphyry belt, which, with the same strike as the other 

 beds — about 20° W. of N. — is met with at Yass Junction, and 

 forms the high cliffs of " Hibernia Crescent," just above the Hume 

 Bridge at Yass. At this locality the porphyry is roughly columnar. 

 (Plates IV. and V.), and has dykes of a creamy coloured spherulitic 

 quartz porphyry running through the main mass. 



Yass Beds. — A few chains above the Hume Bridge at Yass 

 the No. 2 porphyry belt dips beneath the Yass Beds. These 

 consist of numerous layers of shales, sandstones and grits, with a few 

 thin beds of limestone. The beds are very much faulted, good 

 examples being visible near the tramway bridge and in O'Connell 

 Town, near the rifle range. The lowest beds are of shales and mud- 

 stones, apparently fossiliferous. On top of the shales are false 

 bedded sandstones, and sandstone containing clay galls, which in 

 parts look like a shale conglomerate with a sandy matrix. Above 

 these are some coarse grits, and ripple marked and sun-cracked 

 sandstones. Part of this sandstone bed is used for building 

 purposes. The sandstones then give place to more shales and 

 mudstones, containing a few Lingula and fragments of Spirifera. At 

 the top of these beds is a thin layer of calcareous sandstone, with 

 the upper surface studded with what look very much like the 

 carapaces of Ceratiocaris. Above this layer is about 20 feet of thin 

 laminated calcareous mudstones and sandstones, containing 

 undeterminable organic markings, sun cracks, and what appear to 

 be crustacean tracks. 



Then follows a thin layer of mudstone containing a branching 

 Favosites, which seems peculiar to this bed. It averages about 

 half-an-inch in diameter, and I traced one specimen more than a 

 foot in length, giving off several branches. About a foot above 

 this bed are found great numbers of a brachiopod embedded in 

 shale which are probably referable to Spirifera plicatella. L. sp. ; 

 also Loxonema and Grammysia. These are followed by another 

 layer of casts of Ceratiocaris (?) 



Six inches higher is a thin micaceous sandstone, containing a 

 remarkable deposit of Leperditia shearsbii, Chapman, and Rhom- 

 hopteria (Pterinea) laminosa, de Kon., together with Loxonema (?) 

 strangtdata, and some distorted shells of Spirifera plicatella {}) This 

 layer is very easily picked out in the quarry, of which an illustration 

 is given in Plate VI, as the Leperditia and Rhombopteria are crowded 

 into a thickness of only about an inch at this particular spot, 

 although they occur scattered singly in the beds immediately above 



1 Shearsby, A. J. : " On a Bed of Fossiliferous Tufi," etc., Proc. Linn. Soc. N.S.W., 1905, p. 282. 



