GEOLOGY OF THE JENOLAN CAVES DISTRICT. 357 



contorted, and the quartz crystals highly shattered, the 

 detached fragments of the same crystal often showing 

 different optical extinction. The felspar crystals also 

 exhibit strain-shadowing and fracturing. What has been 

 referred to in the above description as flow structure may 

 be due in a large part to foliation resulting from dynamic 

 metamorphism, although from the manner in which the 

 groundmass swirls around the phenocrysts as seen in some 

 sections from this part of the occurrence, there can be no 

 doubt that this structure is due in part to the flow of the 

 groundmass before consolidation. 



Specimens collected from McEwan's Greek, at the place 

 marked D on the geological map (Plate LV), and which 

 appears to be an undoubted continuation of the occurrence 

 near the Grand Arch, show a well marked flow structure 

 in the hand specimen. This rock under the microscope 

 is seen to be almost coarsely microcrystalline with an 

 allotriomorphic-granular fabric; phenocrysts are typically 

 absent. Veins of secondary quartz are numerous, cutting 

 across the lines of flow. This phase of the rock has the 

 appearance of a rhyolite which has undergone much 

 devitrification, with subsequent intense silicification, the 

 latter generally along lines of original flow. The phase 

 described from near the Grand Arch has more the appear- 

 ance, both in hand specimens and under the microscope^ 

 of a typical quartz-porphyry. 



Si0 2 .. 

 A1 2 3 

 Fe 2 3 

 FeO .. 

 MgO ... 



CaO .. 



Na,0 



Chemical 



Composition. 





Per Cent. 





Per Cent. 



7418 



H 3 (100° 



C.) 0-15 



11-15 



H 2 (100° 



C. + ) 1-27 



0-90 



CO, ... 



... 2-00 



1-31 



Ti0 2 ... 



0-25 



0-62 



Zr0 2 ... 



... absent 



1-35. 



P 3 5 ... 



0-15 



1-00 



so 3 ... 



... trace* 



5-86 



CI 



... trace* 



