236 J. J. H. TEALL—PETROLOGICAL NOTES ON 
external portions of individual grains—that is, to those portions 
which were in contact with the consolidating magma (Pl. XIII. 
fig. 1). 
The porphyritic felspars frequently contain large numbers of 
minute (004 mm.) elliptical inclusions of a brownish glass, and 
sometimes large, irregular, and nearly opaque masses of the ground- 
paste. Augite granules are not uncommon in the peripheral zone. 
Under crossed Nicols two types of twinning may be recognized, the 
lamelle of the prevailing type being usually broad and frequently 
terminating in the centre of a crystal or crystalline grain. 
The microscopic structure of the amygdaloids has been already 
described in connexion with the Hebburn dyke. 
An experiment was made to ascertain the relative fusibility of 
the ground-mass of this rock and the porphyritic crystals. Several 
pounds weight were placed in a crucible, and subjected to the heat 
of a brass furnace. It was found that this was sufficient to melt 
the ground-mass; but the large felspars remained intact, so that 
after rapid cooling they were found imbedded in a brown isotropic 
glass. In asecond experiment the cooling was prolonged for fourteen 
hours, and it was then ascertained that dense spherulites similar 
to those of the tachylite (? hyalomelan) of Sababurg had developed 
themselves, and that a similar mode of devitrification had taken 
place round each of the porphyritic crystals. 
Brunton DYKE. 
Interature. 
Lzzovr, Prof. G. A. Geology of Northumberland. 
Ordnance Map 106 N.E. 
The course of this dyke in thus described by Prof. Lebour :— The 
Brunton dyke, known in west Allendale, near Whitfield, crosses the 
South Tyne first to the west of Haydon Bridge, then between that 
little town and Wharnley, and lastly to the east of the latter place ; 
it crosses the North Tyne near Wall, is well exposed by St. Oswald’s 
Chapel, near Brunton, and is last seen in the Bingfield Burn on the 
east side of the Watling Street. Two small dykes run close to and 
parallel with this one near its easternmost extremity. Ofthese one 
is well seen in the Bingfield Burn, and the other in the bed of the 
Pont, where that little stream crosses the Watling Street.” 
The general trend of the main dyke appears to be nearly N.E. 
and S.W. On Feb. 2, 1882, I had the opportunity of examining 
two of the dykes above referred to in company with Mr. P’Anson. 
We were directed to the spot by Prof. Lebour. They are exposed 
in the small burn which crosses the Watling Street about halfa mile 
W.S.W. of Bingfield. Leaving the road at this point and working 
up the stream for a quarter of a mile, or rather less, we see the 
main dyke exposed on the south side of the stream, where it is in- 
trusive in encrinital limestone. The main mass is composed of a 
