MICROSCOPICAL CHARACTERS OF THE WHIN SILL. 657 
a single instance of this mineral in the Whin Sill, although Mr. 
Allport mentions it as occurring in the rock from Ward’s Hill*, near 
Rothbury. 
Outside this country, the nearest allies to our rock that I can 
find are certain Swedish diabases described by Tornebohm +, and the 
great masses of trap which occur as dykes and intrusive sheets in 
the Triassico-Jurassic strata of the Eastern States of North America t: 
I have a good series of the American rocks from New Jersey and 
Connecticut, and both macroscopically and microscopically they are 
in many cases identical with the Whin Sill. In mode of occurrence 
and chemical composition there is also great similarity. Lime- 
felspar, however, appears to be more abundant in the American rocks. 
Dr. Hawes separated the felspar substance, extracted from a rock 
from Jersey City, into two portions, one having Sp. Gr. > 2°69, the 
other < 2°69. The former, on analysis, proved to be labradorite, 
the latter andesine. 
It is not a little interesting to observe, near the opposite shores 
of the Atlantic, rocks with so many points of close resemblance. 
EXPLANATION OF PLATE XXTX. 
Fig. 1. A row of twinned crystals of the dominant pyroxene, cut at right 
angles to the ¢ axis, from a coarse variety of the Whin Sill near 
Tyne Head. The forms and mutual relations of the crystals are 
accurately represented. In the actual specimen the interspaces are 
filled with decomposed felspar and a little micro-pegmatite. If we 
speak of the sections as approximately rectangular for purposes of 
reference, then the optic axial plane lies parallel with the short side 
of the rectangle, which consequently represents the clinopinacoid. 
The long side of the rectangle is the ortho-pinacoid, and the 
prismatic faces are represented by the slight truncations of the. 
angles. The crystals are seen to be twinned with the ortho-pinacoid 
for the face of composition. xX 24. 
2. Section of twin crystal of the same pyroxene cut approximately parallel 
to the clino-pinacoid. The striations of the two halves make with 
the twinning-line angles of 76° and 75° respectively. The mean of 
these is 74° 30', and the angle 6 of augite, as given in Naumann- 
Zirkel’s Mineralogie, is 74° 11’. The two halves extinguish at angles 
of 42° 35' and 40° 35’ respectively. x 80. 
3. Sections of two imperfect pyroxenes with marginal hornblende. The 
external form of the hornblende and the characteristic cleavages are 
well shown. ‘The minerals in contact with the hornblende are chlo- 
rite and quartz. X24. . 
4, 5,6, 7§. Forms of the magnetic titaniferous iron-oxide. The dotted 
spaces represent leucoxene. The mode of alteration reveals the 
structure of the substance. It consists of a framework of ilmenite 
lamellz with the interspaces occupied by a different substance, pro- 
bably magnetite. Middleton. X40. 
* Quart. Journ. Geol. Soe. vol. xxx. p. 552. 
+t Neues Jahrbuch, 1877, p. 258. 
t W. Hawes, “Trap Rocks of the Connecticut Valley,” Amer. J. Science, 
1875, p. 185. 
§ From a section kindly lent by Prof. Lebour. 
