GEOLOGY OF THE JENOLAN CAVES DISTRICT. 371 



alteration product of the ferro-magnesian minerals, felspar 

 and ground mass. 



Epidote is present both as an alteration product of the 

 phenocrysts and of the groundmass. It is of a yellowish- 

 green colour with fair pleochroism, and occurs as crystals 

 with the typical columnar structure, and as irregular patches 

 and grains. It is more plentiful in some slides than in 

 others. 



The groundmass. — This is generally much altered and 

 does not admit of detailed description. It is of a yellowish 

 or brownish clouded colour, changing to pale green where 

 chlorite is plentiful. It may be briefly described as a 

 micro-crystalline mixture of felspar, orthoclase and plagio- 

 clase, the former predominating, and quartz with the 

 secondary products chlorite, kaolin, calcite and epidote. 

 Quartz is fairly plentiful and is more or less interstitial in 

 habit. Where the groundmass of this rock is at all fresh 

 it shows distinct micrographic structure; especially is this 

 the case in slide x 66 where exquisite examples of this 

 structure occur, and in which it constitutes a large part of 

 the groundmass. Frequently the groundmass encloses 

 these patches of micropegmatite, like porphyritic crystals, 

 with irregular boundaries, arranged around a centre and 

 with an average size of about 0*4 mm. but reaching 1 mm. 

 Often the orthoclase is twinned on the Carlsbad law when 

 the structure assumes a delicate feather-like appearance, 

 the composition plane of the orthoclase corresponding to 

 the rib of the feather and the parallel intergrowths of quartz 

 to the barbs. 



This rock with its distinctly holocrystalline groundmass 

 is inclined more to plutonic than volcanic types. In struc- 

 ture it is closely allied to the granophyres of the acid group. 

 With a silica content reaching only 61%, and characterised 

 by a porphyritic soda-lime felspar, it must however, be 

 included in the intermediate group. 



