The Sutherland Volcanic Pipes. 69 



up to a foot in diameter. (1) A dark rock with many garnets visible 

 to the naked eye. Its specific gravity is 3*15. The mineral con- 

 stituents are almost colourless augite, brown hornblende at places 

 intergrown with the augite, a very basic plagioclase, pale pink garnet 

 always surrounded by a dark rim of alteration products, and ilmenite. 

 The structure of the rock is granulitic, and crystalline forms are 

 rarely seen. (2) A black rock with specific gravity 3*23 ; it is 

 composed of pale green augite, strongly coloured brown hornblende, 

 strongly pleochroic brown mica, ilmenite, calcite filling up cavities 

 within the augite, mica and ilmenite, apatite, and a cloudy alteration 

 product. No felspar is visible. The augite and hornblende tend to 

 form aggregates of considerable size from which the other constituents 

 are excluded. There are several intermediate varieties between these 

 two, which appear to be respectively the least and most basic repre- 

 sentatives of one rock mass. (3) A banded black and white granulitic 

 rock. The mineral that appears to be black in a hand specimen is 

 brilliantly green in thin section and it is a chrome-diopside in irregular 

 rounded grains. The white portion of the rock is a cloudy mass 

 (saussurite) of greatly altered felspar. A considerable amount of apatite 

 is present. None of these eclogites or granulitic rocks is known in situ 

 in the Colony, but the more basic black varieties present points of 

 resemblance to rocks that are found as inclusions in the blue ground 

 of the pipes north of the Orange Eiver. 



From the second shaft, a short way to the north-east of the one 

 from which the rocks just described were obtained, there came an 

 agglomerate with a harder matrix than that from the south-western 

 pit. The matrix is less serpentinous and more sandy ; if it were 

 destitute of inclusions it might even be called a calcareous clayey 

 sandstone, but microscopic examination of a thin section reveals the 

 presence of serpentine, perofskite, iron ores, and ilmenite. The in- 

 cluded fragments visible to the naked eye are sandstone, quartzite, 

 shale, augite, mica, hornblende, and ilmenite, all similar to those 

 described from the other shaft ; chrome-diopside is rare ; serpentine 

 pseudomorphs after olivine are not uncommon. Dolerite of the 

 Karroo type is frequently seen in this rock. All these fragments are 

 outlined by a narrow band of altered material. 



The Silver Dam breccias have a close resemblance to the 

 Kimberley blue-ground, from which they chiefly differ in being 

 less serpentinous ; the minerals enstatite, smaragdite, orthite, 

 tourmaline, epidote, and diamond, which occur more or less sparsely 

 in the blue-ground have not been found at Silver Dam. We shall 

 return to this question after the description of the other pipes. 



