OX THE PIEDMOXTITE-RHYOLITE FROM SHIXAXO. 119 



Megascopically, the piedmontite-rhyolite is a light-coloured, com- 

 pact rock of trachytic appearance. The ground-mass is dull-white 

 and encloses phenocrysts, or grains of feldspar and quartz, besides 

 many small aggregates of common grass-green epidote and cherry -red 

 piedmontite. Some of the aggregates look like porphyritic crystals, but 

 on close observation are seen to be really assemblages of minute crystals 

 which have replaced certain original crystals, or occupy fine fissures 

 or drusy cavities of varying size. In some large cavities the aggregate 

 consists of beautiful fine needles of piedmontite radially arranged. 



Under the microscope, the rock is seen to be decomposed through 

 weathering. The micro-holo-crystalline ground-mass, which is a plexus 

 of fine grains of feldspar and quartz, is thoroughly clouded with 

 greyish-brown decompotition products of its own, as well as of those 

 of the porphyritic crystals. The outline of quartz, which occurs in 

 both macro- and micro-porphyritic grains, is rounded in all cases by 

 the corrosive action of the magma. Indentations of the ground-mass 

 into the quartz are not infrequent ; sometimes, many of them are found 

 in a single grain, as shown in Fig. 2. Gas-inclusions are rare, and 

 when found have usually a linear chain-like arrangement. 



Feldspar occurs in the two forms of orthoclase and plagioclase. 

 The former, less in quantity than the latter, is commonly lath-shaped 

 and often exhibits the Carlsbad twinning. The plagioclase is twinned 

 after the albite-type. Both feldspars are more or less kaolinised 

 through decomposition and there is no crystal which is not thus 

 changed. When highly decomposed, the whole of the crystal is stained 

 of a dull-greyish colour, and the poly synthetic twin-lamellaa of the 

 plagioclase are hardly distinguishable. Chloritic matter is also dis- 

 seminated through some of the crystals. The nature of the plagioclase 

 could, therefore, not be determined. 



