J. J. Harris Teall—Cheviot Andesites and Porphyrites. 257 
seen to consist of biotite and felspar in a compact reddish-purple 
matrix. Under the microscope the felspars are seen to be wonder- 
fully fresh and well striated, at any rate for the most part. The 
two sets of lamellz in sections more or less at right angles to the 
twinning plane extinguish within a few degrees of each other, and 
nearly parallel with the longer diameter of the crystal section. The 
other distinctly recognizable and undoubtedly original minerals are 
biotite, magnetite or ilmenite and apatite. Patches of granular and 
spherulitic quartz are also seen here and there in the section. ‘The 
ground-mass is crypto- or micro-crystalline, and contains both opacite 
and ferrite, distributed in a somewhat irregular manner. The ferrite 
is especially abundant in fluidal bands and stripes which curve round 
the larger crystals in a very characteristic manner. Vogelsang' 
describes a similar distribution of ferrite in certain of the Hungarian 
quartz-trachytes. 
In certain parts of the section the ferrite is collected in ill-defined 
spherical masses (cumulites) of uniform size, and where this takes 
place in the presence of opacite, the latter mineral is absent from 
the cumulites. The red bands and patches frequently have the same 
sort of outline that they would have if they were made up of col- 
lections of cumulites, but the ferrite is distributed in a uniform 
manner in their interior portions. 
Tue Turrs aND BRECCIAS. 
Good sections of the bedded tuffs and agglomerates are exposed on 
the left bank of the Alwin, immediately below Allerhope Burn, and 
again at a point half a mile below Kidlandlee Dene, and on the same 
side of the valley. The coarse breccias may be examined in the bed 
of the Alwin, about a quarter of a mile above Windy Haugh, and 
again at Blindburn. Doubtless there are many other equally good 
exposures. 
Fine ash, the Alwin, below Kidlandlee Dene.—The rock is of a 
greenish-grey colour, and the constituent particles are just recog- 
nizable with a hand-lens. ‘The microscope reveals the presence of 
small fragments (lapilli) of andesitic porphyrite and broken felspars, 
in what may be called a ground-mass of somewhat indefinite 
character, doubtless due to the alteration of fine volcanic dust. 
Opacite, ferrite, and viridite are scattered throughout the section. 
Coarse ash, the Alwin, below Kidlandlee Dene.—This specimen shows 
alternating bands of coarser and finer material, the larger fragments 
frequently attaining the size of peas, and sometimes that of beans. 
Under the microscope the larger fragments are seen to consist of the 
typical andesitic porphyrites of the district, and frequently give 
evidence of an original scoriaceous or vesicular character by the 
presence of amygdaloids of green earth and chalcedony. The frag- 
ments are mostly of irregular form, but in one or two instances the 
angles are fairly well rounded. Broken felspar crystals are present 
in this, as in the previous rock, together with a small quantity of 
indistinctly characterized ground-mass. 
1 Die Krystalliten, p. 161. 
DECADE II.—VOL. X.—NO. VI. i 
