MoRfiAN. — llic lyneous Eorks of Hit Wa'ilii Goldfield. 265 



fresher examples an abundance of small feldsj^ar laths crowded together 

 in more or less parallel arrangement is present, and thus the structure is 

 decidedly inclined to be pilotaxitic ; but, since calcite and other alteration- 

 products are invariably present to some extent, the original pilotaxitic (or 

 possibly hyalopilitic) structure is partly obscured. More commonly the 

 groundmass consists mainly of a granular mosaic of quartz^ feldspar, and 

 calcite, with minor amounts of other minerals. 



(6.) Wall-rocks. 



The rocks now to be discussed are from the same localities as those 

 coming under {a), except that, as indicated by the heading, the specimens 

 examined were obtained either within a few feet of the larger veins or were 

 taken from the actual walls of the veins. 



Macroscopically the wall-rocks are evidently more highly altered than 

 those some distance from the veins. They are lighter-coloured, and do 

 not effervesce so freely with hydrochloric acid. Feldspar and, less com- 

 monly, quartz phenocrysts may be observed. Ferro-magnesian minerals, 

 while usually distinguishable, are always intensely affected by decompo- 

 sition. 



Under the microscope the sections show extreme alteration. The feld- 

 spar phenocrysts are nearly all so decomposed that the identification of 

 their original character becomes micertain. In one or two sections fairly 

 fresh binary twins of orthoclase may be recognized. In most of the sections 

 there are feldspars showing traces of repeated albite twinning, and hence 

 it may be concluded that these feldspars were originally plagioclase. In 

 the main, however, albite twinning is absent, and, since the refractive index 

 of any recognizable feldspar is well below that of Canada balsam, the mineral 

 approaches orthoclase or anorthoclase in its characters. The most highly 

 altered feldspars consist of a mosaic of quartz and feldspar, with minor 

 quantities of calcite and " kaolinitic " matter. In some cases the feldspar 

 of the mosaic is evidently secondary, and therefore to be termed " valen- 

 cianite." In other instances, however, its secondary nature is less certain, 

 though appearances are generally quite consistent with such a conclusion. 

 The so-called " kaolinitic " matter mentioned above is strongly suspected 

 to be near sericite in composition, but in the absence of chemical tests its 

 exact nature seems indeterminable. 



Original quartz grains appear exactly as in the rocks coming under 

 the last heading, except that they are in some instances in optical continuity 

 with clearly secondary quartz, a phenomenon not observed in the less- 

 altered country. 



The ferro-magnesian minerals are entirely altered to chloritic material, 

 with a little iron oxide, &c., so that the original species can hardly be more 

 than guessed at. Rhombic pyroxene may perhaps be assumed as having 

 been present. Primary hornblende is doubtfully indicated by the presence 

 of many small magnetite grains in some of the chloritic matter which has 

 outlines possibly referable to amphibole. 



Other original minerals include a little magnetite, often with leucoxene- 

 like alteration (? to siderite), and a few small needles of apatite. A section 

 made from a diamond-drill core obtained in the Grand Jmiction Mine at a 

 depth of nearly 800 ft. from the surface shows a nest of some mineral with 

 the appearance of tridymite. '' 



The groundmass in the sections of wall-rocks is a granular mosaic com- 

 posed largely of quartz, with probably more or less valencianite, a little 



