306 GEOLOOY AND PETROLOGY OF THE GREAT SERPEXTIXB BELT OP N.S.W. 



whieh, however, is probably a local centre of radiation in a region of general 

 east-north-easterly trend of dyke-fissures (Compare 25). Dykes very frequently 

 occupy fault fissures, so that it is probable that the main fault-movements accom- 

 panied or preceded the formation of the dykes. 



The more acid dykes consist of rocks closely resembling the material compos- 

 ing the keratophyTe sills. These are most obvious in the conglomerates and fels- 

 patliic grits, but have not yet been traced through into Burindi Mudstone. The 

 longest of these dykes crosses from the hills west of the head of Anstey's Creek, 

 and may be traced thence across the sills of quartz basalt and through the Kuttun* 

 rocks as far as the slopes overlooking Sandy Creek. The fissure which it occu- 

 pies seems to be continued by the fault truncating, on the south, the pyroxene 

 andesite in the valley. Of the basic dykes, we may note the group of outcrops, 

 which commence in the railway cutting in portion 34, pass through the gap be- 

 tween Cobla and Sugarloaf, and seem to be continued in a dyke crossing Curra- 

 bubula Creek in portion 140 and the main road in portion 319 . The rock in these 

 is generally decomposed, but in the above-mentioned gap is a compound dyke of 

 decomposed basalt containing another dyke in the centre composed of fresh dole- 

 rite. Other fresh specimens of basic dykes have been found, particularly where 

 such traverse well-cemented conglomerate. Very frequently decomposed basic 

 dykes occur in fault fissures, and such fault-dykes traverse the sills of pyroxene 

 andesite. 



Dykes of porphyi-itic dolerite occur abundantly in the Warragundi complex 

 but have not yet been studied in detail. They also occur in the arcuate line of 

 hills that run to the south-west from that volcanic centre. 



With regard to the relative age of these various dykes and sills, little can 

 yet be stated, except that, apart from the cases in which the keratophyre dykes 

 and sills may be contemporaneous, but older than the pyroxene andesites, the dykes 

 are younger than the sills wherever their intersections have been observed. Per- 

 haps here also the ])hase of minor intrusions was one of increasing basicity (Cora- 

 pare 25). Nevertheless, it must be emphasised that much detailed observation of 

 these rocks is yet required, and that the present list of types of rock developed, 

 and mapping of intrusions is far from being exhaustive . 



The Permian ( ?) Glossoptkris Sandstone. 

 These rocks were first noted by Mr. J. E. Carne (18), being pointed out 

 to him and later to ourselves by Mr. Hammond, of Escott Park. They are not 

 present in the area mapped in detail, but occur in Grenfell Parish, and extend 

 along the western side of the railway line for a mile and a half, commencing two 

 miles south of Werris Creek. Mr. Carne noted a dip to the W.S.W. at 20° on 

 the eastern edge of the sandstone, and we observed a dip of like amount in the 

 opposite direction on the western side. The sandstone is thus bent into sympathy 

 with the folding of the Kuttung Beds and lies over the synclinal axis. It is prob- 

 ably not merely a residual mass left by erosion ; its sharp, almost rectilinear 

 boundaries are more in accord with the view that it is an infaulted outlier brought 

 now into relief by differential erosion of the softer basalts about it. The rociv 

 consists of sandstone and a little conglomerate, (|uite different from that of the 

 Kuttung rocks; indeed the sandstone is remarkably similar to that in the New- 

 castle Coal Mea.«UJ'es west of Pokolbin, with which it is tentatively correlated. 

 Like them it contains several species of Glossopterix with Yertchrnria and silici- 

 fied (coniferous?) wood. Mr. Carne, however, suggested that it might be cone- 

 lated with the Creta dial Measures. 



