GEOLOGY OF THE JENOLAN CAVES DISTRICT. 349 



The porphyritic type from this locality contains biotite in 

 addition to augite, and has a decidedly lamprophyric struc- 

 ture and could be called a true lamprophyre. 



We have been unable to find outcrops between the two 

 localities already referred to, although fragments have been 

 found in one or two places near the Tarana Road in the 

 surface soil along the line of strike; there is, however, 

 little doubt that they form part of a continuous intrusion, 

 the absence of good outcrops being due to the ready decom- 

 position of the rock. 



At first it was thought that the patches of porphyritic 

 andesite occurring in the non-porphyritic andesite were 

 xenoliths, but the occurrence of the two rock types in two 

 separate dykes at locality A, suggests that the occurrence 

 at the Cave House is a composite dyke in which there was 

 first intruded the porphyritic type of andesite, to be followed 

 later by the non-porphyritic type, the later intrusion break- 

 ing up and partly reabsorbing the already consolidated 

 earlier intrusion. At the more northerly locality (A) the 

 combined bulk of the two intrusions is much smaller, and 

 the second intrusion did not keep to the same channel as 

 the earlier one, and thus formed a separate dyke. A 

 detailed petrological description of these andesites with 

 analyses is given in Part II, and it will be seen that they 

 are sufficiently alike to have originated from a common 

 magma. 



The age of this intrusion is difficult to determine ; it 

 intrudes the radiolarian cherts and is therefore newer than 

 them, but is not known to intrude the Silurian strata. It is 

 practically in contact with the Cave limestone just south 

 of the Cave House, yet no included fragments of limestone 

 have yet been found, whereas included fragments of the 

 radiolarian cherts are common. Included fragments of 

 limestone are quite common in the quartz-porphyrites and 



