GEOLOGY OF THE JENOLA.N CAVES DISTRICT. 345 



siderable resemblance to this rock in the hand specimen, 

 but they differ from it considerably in their chemical and 

 petrological character. In the case of these latter rocks, 

 (porphyrites and felsites), there can be no doubt of their 

 intrusive nature; numerous veins run out from them into 

 the adjoining slates, while abundant fragments of the slate 

 occur embedded in the igneous rock at their contacts. No 

 such evidence has been found in the case of the rhyolite- 

 porphyry, although it is true that the contacts so far 

 observed are obscure. In the case of the porphyrites and 

 felsites, the slates have been so much hardened and in- 

 durated at their contacts, that weathering has been resisted 

 and good sections of these contacts are given in the road 

 cuttings, whereas in the case of the rhyolite porphyry the 

 slates at the contact have weathered so readily that the 

 actual junction cannot be seen. Along the hill sides and 

 stream channels innumerable boulders of porphyrite with 

 included fragments of indurated slate may be seen; but no 

 boulders of the rhyolite porphyry with such inclusions have 

 yet been found. 



The chemical composition of the rhyolite porphyry in 

 comparison with that of the other igneous rocks of the 

 district is given in Part II ; it will be seen that it differs 

 from the intrusive quartz porphyrites and felsites in (a) 

 being more acidic, (b) in the proportion of the alkalis 

 present, potash preponderating over soda, whereas in the 

 other rocks the reverse is the case. 



If the rhyolite-porphyry be a contemporaneous lava flow, 

 it would of necessity have been subjected to the same 

 folding forces as the Silurian sediments, and one might 

 expect a repetition of its outcrop in the eastern leg of the 

 anticline; this repetition does occur in the actual position 

 one might expect to find it, as shown in the geological 

 section. Specimens from this occurrence have such similar 



