GEOLOGY OF YASS DISTRICT. 113 



rolled beach deposits such as are common in the Hume Beds, which 

 are met with lower down the creek near Bowning. 



The commonest fossils found at Wargeila are : — Sponge 

 spicules: Amplexus[?), Cyathopyhllum shear sbii, Eth. fil. M.S. ; Cya- 

 thophyllitm, Diphyp/iylliini, sp., Cystiphyllmii, Rhizophyllum 

 robiistum. Shearsby; R. interpiinctatum, de Kon; Tryplasma 

 derringnUciisks, Eth fil. ; Favosites, Pachypora, Alveolites, Heliolites 

 vUerslincta. Syringopora (?), Stromatopora, Pisocrimis vassensis, 

 Eth. fil. ; Fenestella, Lingula, Orthis, Orthothetes shearsbn, 

 Dun; Spirijer plicatelliis, Meristina{?) australis, Dun ; {?)Rhyn- 

 chotreta, Camarotoechia, Atrypa reticidaris, Spirifer cff. fimbriatus , 

 Rhoniboptcria, sp. ; {?)Modiolopsis, Loxonema, Orthoceras, {?)Endo- 

 cenis, Encrinurus pnnctatus, Brunnich ; Leperditia shear sbii, Gir- 

 vanella pisoliiica, Wethered(?) is also found encrusting corals and 

 shells. The decomposed limestone yields on being washed sponge 

 spicules and microscopic casts of Orthis, Modiolopsis. and nume- 

 rous undeterminable gasteropods. 



No. 3 Porphyry. — Returning to Yass River, about a quarter of 

 a mile below the Hume bridge, it will be found that the Yass beds 

 dip below a mass of porphyritic rocks, which vary in composition 

 and appearance, but are not fossiliferous like the other beds of 

 porphyry previously noticed. This No. 3 bed of porphyry varies 

 in width and extends in a direction about 20° W. of N., crossing the 

 Yass River about two miles below Yass. Here the course of the 

 ri\-er is through about two miles of variously coloured porphyries, 

 which also forms the bed of a considerable portion of the Derren- 

 gullen Creek, and is met with at Wargeila, overlying the Yass beds 

 there. It does not extend far in a southerly direction, as it pinches 

 out at Yass, in the Rifle Range near the targets, and forms the 

 " wedge-shaped " mass of rock mentioned by Professor David. ^ 

 The lowest bed has the same dip as the underlying fossiliferous 

 strata, and has been looked upon by many as part of an intrusive 

 mass which has forced itself between the bedding planes of the 

 latter, and differing from the main intrusion in being partly com- 

 posed of absorbed sedimentary rocks with which it came in contact. 

 It is greenish-grey in colour, and a hand specimen would be liable 

 to lead one to look upon it as the result of contact metamorphism. 

 A specimen forwarded to Dr. Woolnough from the back of Clifton- 

 wood House, where good undecomposed blocks are exposed in a 

 road cutting, was determined by him as a " quartz porphyry tuff, 

 certainly clastic, and showing not the slightest signs of absorption 

 phenomena." 



This tuff is overlaid by a much coarser-grained rock, varying in 

 colour and texture in different localities. Where it crosses the 

 river it has a decidedly vertical bedding, some of the beds being of 

 a creamy colour, others chocolate and grey. Veins of calcite, from 

 a hair's breadth to an inch or more in thickness, are common, 

 running for considerable distances with the line of strike. Double 



i David, T. W. E. : " Report on the Fossiliferous Beds, Yass," Ann. Rep. Depi. Mines, n'.S.W., 

 1882, p. 148. 



