Geology of High Teesdale. 165 
resistance to the blows of the hammer. The fracture of the finer 
grained varieties is often imperfectly conchoidal, in the coarser 
specimens it is almost always uneven. A newly exposed surface 
is generally of a dark grey colour, and exhibits a granular tex- 
ture, in which the light coloured feldspathic portion of the rock 
is distinctly separated from the brilliant dark crystals of pyroxene. 
In this respect, the Whin-Sill greatly differs from the dykes of 
the Durham coal-field, in which the component elements are so 
intimately blended, that the naked eye can seldom distinguish 
their separation. The feldspathic portion, notwithstanding the 
granular texture of the trap of High Teesdale, seldom appears 
in the form of distinct crystals. When it is exhibited in greatest 
perfection, it may be detached from the general mass in minute 
fragments, which melt with a slight ebullition, in the flame of 
the blowpipe, into a light semi-transparent glass, and it appears, 
in this respect, not to differ from common feldspar. Not unfre- 
quently this portion is made up of minute opaque grains, which 
do not fuse without great difficulty; and in the superficial parts 
of the rock, it rarely passes into an earthy substance, which is 
harsh to the touch, adheres to the tongue, and is infusible. These 
different states of what I have called the feldspathic part of the 
trap, may not always arise out of the progress of decomposition. 
It may, therefore, in some cases admit of doubt, whether the 
white earthy constituent is to be regarded as a true simple 
mineral. 
In all the varieties of the trap of High Teesdale, pyroxene oes 
is very abundant, and it forms, occasionally, the principal con- — 
stituent. It is sometimes amorphous, but generally crystalline, 
and exhibits, through the surface exposed by a recent fracture, 
a number of minute and very brilliant facets of a black or brownish 
black colour, and the foliated structure of the mineral is occa- 
sionally seen even in the small-grained varieties of the rock. 
Among the hard and almost indestructible masses, there may be 
