408 FERGUSON. 



pyiitized, exposed in the bottom of the open cut, back of the Gold Bug 

 mill. Although none of the original minerals are visible to the eye, the 

 microscope shows a large nmnber of broken plagioclase gi'ains, together 

 with rarer augite grains and innumerable specks of black material, pro- 

 bably carbonaceous matter. This shale again suggests local lacustrine 

 conditions. Above the shale are coarse and irregular conglomerates and 

 sandstones, with irregular and angular pebbles and grains, probably result- 

 ing from torrential deposition. The pebbles of the conglomerate, although 

 much decomposed, appear to be derived from the augite andesite. These 

 rocks are cut by a basic dike and small quartz veins. The accompanying 

 mineralization will be discussed when the economic geology of the district 

 is considered. 



The basic rocks. — The basic rocks are for the greater part basalts, differ- 

 ing from one another and from the augite andesite in the relative pro- 

 portions of olivine, the greater basicity of the feldspars present and the 

 relative preponderance of the femic minerals. From their scattered dis- 

 tribution it is probable that they occur in dikes cutting the andesites and 

 earlier rocks. 



One peculiar type merits more detailed description. This rock, a 

 leucite tephrite, is found in a single outcrop just to the west of the 

 road and about one kilometer due west of the summit of Mount Aroroy. 

 In the hand specimen it is a black, fine-grained porphyritic rock, the most 

 prominent phenocrysts being dark augites generally two or three milli- 

 meters in length, and occurring in great numbers. Large grains of olivine 

 may sometimes be- seen, but this mineral is rare. Very minute, glassy 

 leucites can be detected with the hand lens. Under the microscope the 

 augite shows large, fresh, faintly pleochroic crystals with sharp boundaries, 

 often twinned on 110. It frequently contains inclusions of magnetite 

 grains and the larger magnetites are generally associated with it. In one 

 case a large augite crystal seems broken up into a great number of small 

 grains, and small crystals also are scattered through the ground-mass, 

 the smaller grains showing the most marked pleochroism. The augites, 

 particularly the larger individuals, have clusters of minute, black needles 

 radiating from their sides, most nmnerous near the corners. In all, the 

 augite phenocrysts cover about 30 per cent of the area of the slide. 

 Feldspar is present in small amounts, but is so decomposed as barely to be 

 recognizable. From its low index of refraction it is inferred to be ortho- 

 clase. Next to augite, leucite is the most prominent mineral. It occurs 

 in groups of not more than a dozen crystals in each, the single individuals 

 seldom measuring over 0.35 millimeters in diameter. As a rule, the 

 crystals are fairly clear, but the larger ones often contain small black 

 specks as well as minute grains of augite and small, transparent rods. 

 The smaller crystals are entirely dark between the crossed nicols, but the 

 larger ones show very faint lines of anomalous double refraction. 



