716 
of 650 feet, by corrected Aueroid, above the level of the Road. The rock of which the 
Leap is made up is lithologically a trachyte, and may be generally described, when 
examined with a lens or the unaided eye, as consisting of a light-coloured porous 
matrix, in which crystals of sanidine and minute crystals of what appear to be horn- 
blende are embedded. It is seen * to rest upon black shales, at a point in a 
gully flowdug from the north-west corner of the mouiitaiu three hundred feet above the 
Road ; at the junction of the two the shales for a few inches are slightly hardened. The 
lower portion of the sheet is made up of rudely hexagonal curved columns, the outward 
curvative being northwards. The structure of certain parts of this rock would seem to 
imply that in reality it is a succession of lava-flows of variable thickness. The estimated 
thickness of the sheet is not less than 350 feet.” 
(4) Subaqueous Tuep, Alligator Creek, Saint Helens, AIackat. Sp. Gr. 2'618. 
This is a white porous rock, with reddish oxide of iron in strings and lenticular 
veins. The section is cut transverse thereto. 
Section . — All but quartzless, the grains being very few, small, and sparse, with 
matted plagioclases in the ground-mass, the laths being clouded and ill-formed, measuring 
under '01 inch in length. One or two much altered porphyritic felspars occur, with 
traces of many lamellai. Dusty specks of a mineral polarising in higher colours occur 
throughout the section, but they are too minute to resolve with the |-inch objective 
The highest colour produced between crossed nicols in the ground-mass is slate-grey. 
N.B . — Hydrochloric acid failed to produce efEervescence except where the 
section cut through an iron speck, when a very slight effervescence ensued, making one 
suspect the presence of siderite. 
(5) Mount Jukes. 
This rock is similar to (1) except that the colour is lighter, it being white. 
The felspars glisten when the hand sjiecimen is turned about in the light, owing to the 
cleavage planes of the felspars catching and reflecting the rays. 
Section . — By reflected light there is hardly any ferrite, and the orthoclase is less 
kaolinized. The quartzes limpid, as in (1). The rock is a perfect example of holocrys- 
tallinity. There are but two constituent minerals — orthoclase in Carlsbad twins, dusty 
from kaolinization which is rudely parallel to the cleavages, the latter making angles 
with the plane of composition ; and the quartzes, which have marked dihexahedral 
inclusions, liquid, glassy, and gaseous. In some of the quartzes are moving bubbles. 
Others with the dark margin, occur, denoting gaseous inclusions. The quartzes average 
•01 inch, the smallest being '002 inch, and the orthoclase sometimes '03 inch long by 
‘02 inch or less broad. Tw'o or three jet-black lustrous grains, and a few' longer or rod- 
like of the same substance were not identified. Mr. Maitland says : — 
“ Another denuded wreck of an old volcano is to be found in Mount Jukes, 
some 1,800 feet above sea level, and situated on the bank of Neilson’s Creek, and about 
twenty miles distant from Mackay in a north-westerly direction. The mean specific 
gravity of the rock, from specimens in different parts of the mountain, was found to 
be 2 55. Different parts of the mass present different characters, but generally two 
varieties can be recognised — 
(a) A coarse-grained rock in which a matrix can scarcely be said to exist ; and 
(5) A second in which crystals of sanidine and plagioclase are embedded in a 
microcrystalline base, which, with the aid of a lens, is seen to be made 
up of small crystals and crystalline grains of sanidine and hornblende (?). 
See PI. tC, fig. 1. (A.W.G.) 
