GEOLOGY OF YASS DISTRICT 115 



uppermost bed of quartz })orphyry tuff is succeeded by a lenticular 

 mass of impure limestone— the Bowspring Limestone (Plates vi., 

 vii.), from the name of the locality in which it forms the most 

 prominent feature of the landscape. This consists of a bed of 

 coralline limestone, probably tuffaceous in nature, through which 

 the Yass River has cut its way, leaving a cliff on the right bank of 

 the river about 80 feet high. It is composed of alternate layers of 

 limestone fragments, and calcarous shale or tuff, about three inches 

 thick, which, especially at Booroo Ponds Creek, resembles ruins of 

 ancient walls. This bed, probably originally 100 feet thick at 

 Bowspring, thins out to the south-east to a few feet, and I have not 

 succeeded in tracing it with certainty to the north west. The 

 fossiliferous limestone nodules are very much rounded, pointing to a 

 beach origin ; the matrix in places being as though composed of the 

 denudation products of porphyry. 



At Booroo Ponds Creek the fossils are very much silicilied, and 

 corals, such as Favosites and Heliolites, are remarkable for the ex- 

 cellence of their preservation in this mineral, even to the minutest 

 detail of their structure. Although the majority of the fossils are 

 rounded fragments, great solid masses of coral are also met with, 

 some of which must weigh at least a ton. 



These large coralline blocks are chiefly Diphyphyllinn and a 

 compound Cyathopliyllum, with smaller blocks of a new species of 

 Sponoophyllum with corallites up to two inches in diameter. ^'>Mng 

 mostly fragmental the fossils are often indeterminable, the follow- 

 ing being the chief representatives : — Favosites, at least 3 spp. ; 

 Heliolites, at least 3 spp. ; Cyathophyllum shear shii, Eth. fil. M.S. ; 

 Cyathophyllum, sp. ; Spongophylhim, 2 spp. ; Stromatopora, sp. ; 

 D phyphylhmi flexiiosum, Linn. sp. ; Tryplasma lonsdalei, Eth. fil. ; 

 Cystiphyllinn; Canites: (?) Murchisonia; Atrypa reticularis: crinoid 

 remains ; sponge remains ; Encrintirns ; Orthoceras. 



The Bowspring Limestone is succeeded by the Barrandella 

 Shales^ which consist of highly fossiliferous shales and mudstones, 

 with ^'crj' thin bands of limestone. These shales contain an enor- 

 mous number of fossils, some of which have the appearance of 

 having been derived from coral reefs in the vicinity, being very 

 much rolled and worn before being finally deposited. Extensive 

 deposits of crinoid stems and ossicles are found in this shale, but I 

 have never succeeded in finding rejnains of any of the calyces, so 

 that so far the genera to which the stems, etc., belong to have not 

 been determined. The Barrandella Shales are also very prominent 

 in Derrengullen and Limestone Creeks, where, especially in the 

 latter locality, much better preserved fossils may be obtained, the 

 shales being softer to work and less friable than at Hatton's Corner, 

 An incomplete list of fossils to be obtained in the Barrandella 

 shales is as under : — 



Bythotrephis tennis, J. Hall ; ReceptacuHtcs australis, Salter (?) ; 

 Ischadites lindstrcemi, Hinde ; (?) Carpospongia, also a small 

 Hexactinellid (?) sponge encrusting corals; Palatocychis (?), 



1 Etheridge, Junr., R. : Records Austral. Museum, Vol. V., 1904, p. 289. 



