540 W. AV. WATTS OK THE IGNEOUS AND 



may be intrusive and take the place of the pipe of the old volcano. 

 Of this, however, I have no further evidence than the sudden disap- 

 pearance of the lava to the S.W. We might expect such rocks to 

 be more highly crystalline than the others ; but this need by no 

 means necessarily be the case, for the amygdaloidal character of the 

 more basic rocks, shortly to be described, shows that those parts at 

 present exposed could not have been subjected to very great pressure, 

 but probably cooled near the surface. 



Macroscopically these rocks are dark grey or dull greenish in colour, 

 weathering light brown or white, sharply jointed, sometimes columnar 

 (as in the N-W. flank of the hill, where the columns are from 2 feet 

 to 18 inches across) and often traversed by platy joints at right angles 

 to the columns. They reveal a dead ground-mass with small por- 

 phyritic crystals of felspar — pink, white, or green, rarely more than 

 •1 inch in length — and black crystals of pyroxene, generally smaller : 

 these are well shown on polished surfaces. Often the felspars look 

 broken and rounded, and tempt one to think there may be ash-beds 

 amongst the lavas ; but though I have most carefully searched for such 

 beds in the field and with the microscope, I have been unable to 

 obtain conclusive evidence on this point, even the most fragmental- 

 looking to the eye giving evidence under the microscope that they 

 are merely lavas in which the crystals have been a little knocked 

 about ; and Prof. Bonney, who has most kindly examined several of 

 my most typical specimens, entirely concurs in this conclusion. 

 Microscopically, these rocks present only varietal differences, so that 

 general descriptions will suffice. 



1. Felspar. The smaller crystals have frequently perfect angles 

 and edges ; the larger are often broken or incomplete, at least at one 

 end. There are generally two kinds present : — first, large crj^stals 

 showing polysynthetic twinning; these are generally in the minority; 

 but when this is the case the felspars have undergone much altera- 

 tion, and this may havemasked the twinning; secondly, singly twinned 

 or untwinned crystals, sometimes quite like sanidine in their glassy 

 clearness, cracks, and inclusions ; but Prof. Bonney thinks these are 

 generally plagioclase : probably both kinds are only labradorite. They 

 are both much altered, chiefly to kaolin; and often this alteration 

 following the almost rectangular cleavage-cracks, has isolated 

 small spherules of unaltered mineral. Inclusions are very common, 

 and usually the matter of the inclusions closely resembles the 

 base of the rock ; and even when it differs, it is seen to be only in a 

 slightly different state of alteration, for transitions may be traced. 

 In the specimen, which looks, when weathered, most like an ash (from 

 the S.E. crags, 300 feet* from the summit), the inlets of the base are 

 most obviously connected with growth ; for from their shape they 

 could not be accidental fractures, as a glance at fig. 4 will show. In a 

 specimen taken at the northern end, 400 feet from the summit (fig. 3), 

 Prof. Bonney noticed a little epidote and serpentinous aggregates 

 enclosed in the felspar, and the latter are pretty common in other 

 specimens. 



* These, and the other distances given, express vertical height. 



