MICROSCOPICAL CHARACTERS OF THE WHIN SILL. 643 
only at the junctions, and is never more than a few inches in thick- 
ness. It is especially interesting because it exhibits a very distinct 
type of micro-structure. The coarse-grained varicties occur only 
where the rock attains a very considerable development, as, for 
instance, at Cauldron Snout. There is one very remarkable variety 
in which crystals of pyroxene measuring an inch in length are by 
no means uncommon. I have observed it in two localities, viz. 
half a mile south of Tyne Head and at Cauldron Snout. It 
occurs in the form of irregular masses in the coarser varieties of 
normal whin, Speaking of this variety, Prof. Sedgwick says * :— 
** Among the hard and almost indestructible masses there may be 
found a few concretions or irregular veins of a much coarser and 
more decomposing variety of rock in which the crystals of pyroxene 
are large and abundant. This mineral in such cases often puts on 
the form of irregular prisms or lengthened tabular crystals, the 
planes of which are bent and undulating.” Mr. Phillips examined 
these long crystals for Prof. Sedgwick, and ascertained “ that they 
cleave easiest parallel to the plane P [> P Naumann], which is 
uncommon, and that the broad surfaces of the long crystals are 
not primary planes but represent the plane h [ o P ow Naumann.” 
These determinations of Mr. Phillips will be verified later on. The 
junctions between the very coarse rock here referred to and the 
more normal whin are remarkably abrupt, notwithstanding the fact 
that the two varieties have the same mineralogical composition. 
If we leave out of account one or two exceptional varieties, the 
statement holds good that the rock of the Whin Sill is remarkably 
uniform in general aspect. Its specific gravity is also fairly con- 
stant, as will be seen by the following list of determinations 7 :— 
poh bankenear rae ouch!» ie.r4 6 Aine 2:924 
itonehoughton near Almwick. .. 5.5.55. 15 2-905 
Greenhead Quarry near Haltwhistle ...... 2°G45 
inaverasitonel Gimeno AMMO, Sh566ch so 5ccc 2°945 
Graces near Doureovyicus scr. 665.556 «teres 2-944 
Mindoletom in: Neesdale so. agi. se apie aes 2:959 ~ 
Amygdaloidal varieties are not common, and where they do occur, 
as at the Harkess Rocks near Bamburgh, they are associated with 
other peculiarities which lead one to suspect that the rock has con- 
solidated under conditions somewhat different from those which 
prevailed during the formation of the normal whin. 
The only constituents distinctly recognizable with the naked eye 
or with the hand-lens are pyroxene, a light-coloured substance 
which is presumably either felspar or the result of its alteration, and 
a few specks of pyrites. Under the microscope a much longer list 
of minerals can be made out. The essential constituents are seen to 
comprise plagioclase of one or more species, and generally more or 
* Cambr. Phil. Trans. vol. ii. 166. 
+ Mr. Clough (Geol. Mag. dec. II. vol. vii. p. 442), gives the Sp. Gr. of a 
specimen from Tinkler’s Syke as 2°82, and that of another from Teward’s 
Bridge, near Forest Church, as 2'84, 
