BY W. G. WOOLNOUGII. 783 



dykes, and passing into granite-porphyry, and quartz-porphyry 

 at its margins. Extensive Tertiary basaltic flows occur in the 

 eastern part of the area, and with them are associated plant- 

 bearing tuffs and bauxites. 



The problems connected with the intrusion of the grano- 

 diorite, and with its contact-effects are many and varied, so that 

 the district is one of exceptional geological interest. 



Ordovician. 



Ordovician rocks were first recorded from this district by Mr. 

 J. E. Carne, F.G.S., Assistant Government Geologist.* The 

 point at which he met with them was near the Tolwong Mine, on 

 the banks of the Shoalhaven River, about 8 miles S.8,E. of 

 Marulan. 



Subsequently an extensive outcrop was found by the author 

 at the Razorback, a long spur extending southwards between the 

 Shoalhaven River and its tributary, Barber's Creek. Hence the 

 rocks can be traced in a northerly direction to within half a mile 

 of the Great Southern Railway line at Morrice's Siding. At this 

 point they disappear beneath the Permo-Carboniferous sandstones. 

 The most conspicuous hill-feature in the district, the Trig. Station, 

 Ballanya, is composed of similar rocks, so that a north and south 

 extension of at least eight miles is indicated for the formation. 

 On the east, they are unconformably covered by Permo-Carbon- 

 iferous rocks at the northern end. Further south, in the very 

 rugged country forming the Shoalhaven Gorge, they seem to pass 

 into slaty rocks in which no fossils have been discovered. On 

 the west, the area of the Ordovician rocks is limited by the 

 grano-diorite towards the north and by the Silurian stiata 

 towards the south. 



The chief rock-type is a black carbonaceous shale exactly 

 similar, lithologically, to rocks of this age in other parts of New 

 South Wales and Victoria. These shales pass into quartzites, 



* "The Copper Mining Industry, &c., of N. IS. Wales." Mineral Re- 

 cources No. 6, Geol. Survey of N. S. Wales, 1908, p. 366. 



