GEOLOGY OF THE JENOLAN CAVES DISTRICT. 351 



of red felspar and quartz, notably on the track down Jenolan 

 Creek at the place marked E on the map. The contact of 

 this rock with the Silurian slates which it intrudes is well 

 shown in the road cuttings on the Mount Victoria road 

 (at G on the map). Here numerous tongues of the igneous 

 rock may be seen running out into the slate, while numer- 

 ous fragments of the slates may be seen entirely surrounded 

 by the porphyrite, the whole giving the appearance, at first 

 sight of a coarse volcanic breccia ; the mixed rock in this 

 marginal zone may be referred to as a contact breccia. 

 The included fragments of slate have, for the most part, 

 been well rounded by corrosion, and the slate has been 

 indurated and hardened into a porcellanite or lydian stone. 



The green porphyrite, as the name implies, has a greenish 

 colour, particularly when slightly weathered, the colour 

 being due to the presence of secondary chlorite. This rock 

 is usually crowded with small phenocrysts of quartz and 

 felspar, and when weathered looks not unlike a coarse 

 sandstone. Similar intrusive contacts occur to those 

 described in connection with the red porphyrite. 



Both of these rocks are traversed by numerous irregular 

 but closely set joints, and when weathering break up into 

 small angular blocks. They both occur as large irregular 

 dykes conforming in general direction to the strike of the 

 Silurian slate, although not entirely so. They are related 

 in composition to the tonalites which occur in the great 

 bathylith of the Kanimbla Valley. This bathylith, which 

 contains granites, tonalites as quartz-augite-diorites, 

 extends along the valley of Oaky Greek to within a few 

 miles of the Jenolan Oaves, and these quartz porphyrites 

 are probably apophyses of this bathylith. It will be noticed 

 on referring to the geological map that the intrusion of 

 pink porphyrite is separated from a neighbouring intrusion 

 of green porphyrite by a long narrow belt of slates ; it would 



