The Sutherland Volcanic Pipes. 65 



separation of Nos. V. and VIII., and of these two from VI. and VII., 

 but the outcrops represented on the plan by the two latter may 

 belong to one pipe. 



The rocks from these outcrops are mainly highly vesicular glass ; 

 when fresh they are dark in colour and somewhat banded. The 

 vesicles are filled with calcite, analcite, and natrolite, besides other 

 minerals that have not been determined. Quartz is not present in 

 the vesicles, and only occurs in these rocks as grains of sand derived 

 from sedimentary strata. In the fresher specimens the glass is 

 isotropic and unaltered ; in it lie large porphyritic crystals of olivine 

 which are now entirely replaced by serpentine ; a greatly altered 

 fine-grained rock adheres to several of the serpentine pseudomorphs 

 as though they really belonged to a rock of which fragments are 

 enclosed by the glassy lavas. The original nature of this enclosed 

 rock has not been made out. The glass also contains a mineral 

 that occurs in small rectangular and hexagonal sections ; the rec- 

 tangular sections have straight extinction and the hexagonal ones 

 are isotropic ; both the refraction and double refraction are low. 

 This mineral has the optical character of nepheline, but it does not 

 give the chemical reactions characteristic of that species. It is 

 frequently altered to a mass of greenish yellow granules. Very 

 small crystals of augite, much magnetite, and minute needles of a 

 doubly refracting mineral are also present. In the absence of a 

 chemical analysis of the rock, it is impossible to decide upon its nature, 

 but it is evidently a very basic glass. It has a strong resemblance to 

 a thin glassy dyke found by Mr. Schwarz * some years ago at Kook 

 Fontein under the Nieuweveld. 



The only outcrop that remains to be mentioned is that of the 

 curved dyke marked No. II. on the plan. It encircles the group of 

 vents already described on the north and east. It is about 1J 

 miles long and 700 feet wide across the forked eastern end, else- 

 where its maximum width is 350 feet. In places east of the vents 

 the dyke narrows down to a few feet. No tuff or ashy material has 

 been met with along its course, and it appears to be a dyke rising 

 more or less vertically through the sedimentary rocks. The rock is 

 a melilite-basalt very like that in No. I. pipe, but it contains rather 

 more cracks filled with zeolites. The ground mass is a felted mass 

 of fibres with small crystals and grains of perofskite, magnetite or 

 ilmenite, augite, flakes of pale biotite, and many crystals of melilite. 

 Much of the melilite is partially altered to a fibrous mineral which 

 polarises rather brightly when the fibres lie parallel, but in the more 

 advanced stages of alteration the pseudomorphs become indistin- 

 * Ann. Kep. Geol. Comm. for 1896, p. 18. 



