The Sutherland Volcanic Pipes. 67 



shaft there is melilite-basalt, dull greenish-black in colour, with 

 large crystals of dark mica up to half an inch in diameter, and 

 smaller green crystals of olivine are clearly seen on a freshly broken 

 surface. It effervesces freely with dilute acid owing to the abundance 

 of calcite. Under the microscope the rock is seen to consist of 

 olivine and a very pale biotite set in a mass of melilite laths, mag- 

 netite, perofskite, and grains of calcite ; a comparatively small 

 quantity of fibrous matter, resembling the ground-mass fibres in 

 the Sutherland Commonage rocks, is also present and may represent 

 a former glassy basis. The melilite crystals are arranged roughly 

 parallel to each other and to the long axes of many of the olivine 

 crystals, thus giving the rock a distinct flow-structure. The melilite 

 crystals average *1 mm. in width and about -01 mm. in thickness; 

 the mineral is colourless or very slightly tinted yellowish-pink. The 

 peg-structure is well developed. The melilite is attacked by hydro- 

 chloric acid with separation of gelatinous silica. In the character of its 

 double refraction, which is positive, it differs from the melilite known 

 from the other melilite-basalts of the Colony. The pleochroism of 

 the pale brown mica has a most remarkable peculiarity, the greatest 

 absorption of light takes place when the traces of the basal cleavage 

 planes lie perpendicularly to the short diagonal of the polariser. 

 It is rather more strongly pleochroic than the biotite in the Suther- 

 land pipes. In the strength of its double refraction it resembles 

 the latter. The olivine is usually fresh, but alteration to serpentine 

 has taken place along some cleavage cracks ; it encloses patches of 

 ground-mass, flakes of mica, and magnetite, but not melilite or 

 perofskite. Both magnetite and perofskite are often arranged as a 

 border to the olivine crystals. The abundance of calcite in this rock 

 is difficult to account for. The constituents mentioned above are 

 very little altered, and the melilite, which contains more lime than 

 any other constituent except the calcite, shows no sign of alteration. 

 It is unlikely that the glass basis contained so much lime as to give 

 rise to the amount of calcite present. The surrounding Beaufort 

 beds do not contain much calcareous matter. It is possible that 

 water containing carbonate of lime in solution made its way through 

 the pipe in an upward direction after the activity at the vent ceased, 

 but in this case it is remarkable that so little change was induced in 

 the older constituents of the rock. The calcite does not fill spaces 

 of the nature of steam holes, but occurs in small grains with partial 

 crystalline boundaries scattered throughout the rock. But for the 

 difficulty of imagining calcite to form from a molten siliceous magma 

 the mineral in the Matjes Fontein pipe might well be considered to 

 be an original constituent. 



