150 J.d. Harris Teall—Cheviot Andesites and Porphyrites. 
as follows. The right-hand portion of the section is divided into 
three tolerably equal bands. The lower third consists of very broad 
bands of the a set, with two or three narrow bands of the 6b set; 
the middle third consists of broad bands of the 6 set, traversed by 
extremely narrow bands of the a set; the upper third is divided 
into two halves, the lower half consisting of broad bands of the 
a set, divided by fine lines of the b set, and the upper half having 
this condition of thingsreversed. The bands here referred to are not 
~ continuous along the whole length of the section. So far as my very 
limited observation has gone, I have found this type of twinning 
characteristic of felspars, which give the extinction angles of labra- 
dorite in cleavage flakes parallel with the basal plane. 
Immediately to the north of the felspar section just described there 
occurs another containing inclusions of the ground-mass arranged 
in a zonal manner round the edge of the crystal section; and to the 
south there occurs a felspar reduced to the condition of a skeleton by 
the number of inclusions which it contains. In this case the bulk of 
the foreign matter must be greater than that of the felspar substance, 
and yet the felspar has impressed its character on the compound mass. 
One is reminded of the masses of Fontainebleau sandstone, on which 
calcite has impressed its characteristic crystalline form. This portion 
of the Plate should be examined through a lens. 
Rather more than half-way between the central felspar grain first 
referred to and the top of the Plate there occurs a somewhat elon- 
gated rectangular section lying with its longer axis inclined a little 
north of east and south of west. It shows a fibrous structure. This 
is a longitudinal section of a somewhat altered crystal of the rhombic 
pyroxene. The fibrous structure is due to the fact that the altered 
and unaltered portions lie side by side in bands which run, roughly 
speaking, parallel with the longer edge of the section. The altered 
portions give aggregate polarization which may be best seen when 
the unaltered portions are dark. The black patch below, and slightly 
to the right of the section just described, is a more highly altered 
crystal of the same mineral. Another similar section may be seen 
a little above the largest porphyritic felspar. At the bottom of the 
figure and intersected by the margin is a fragment of an altered 
pyroxene, surrounded by a dark ring due to the segregation of iron 
oxides. Magnetite occurs, mostly as opaque grains, and may be re- 
cognized in the Plate. The ground-mass of the slide is composed of 
small felspars together with numerous specks and grains of opacite 
(? magnetite) and ferrite (? hematite) imbedded in a tolerably clear 
crypto-crystalline base. I doubt whether any true isotropic glass 
occurs in this rock. Apatite occurs in comparative abundance. 
Roadside, half a mile above Shillnoor Farm, Coquet.—At the point 
in question the road makes a slight turn to the right, and on the 
north side there occurs an exposure of dark purple porphyrite. The 
ground-mass is compact, and throughout it are scattered crystals of 
plagioclase and ill-defined greenish patches resulting from the altera- 
tion of pyroxene. Sp. Gr. 256. The following is an analysis of 
this rock by Mr. Waller, of Birmingham :— 
