Geology of High Teesdale. 167 
of the rock almost entirely disappears. This change in external 
character appears to arise only from the different size and arrange- 
ment of the same component elements. 
Minute fragments of all the preceding varieties of the rock, 
fuse readily into a bright glass bead of an uniform black colour. 
On this circumstance alone, some mineralogists have attempted to 
found a distinction between trap of the kind I am now describing, 
and greenstone, i. e. a granular compound of feldspar and horn- 
blende. 
Every thing which has been hitherto said on the trap of 
Teesdale, is derived from observations made at those localities 
where the beds are of considerable thickness. At the castern ter- 
mination of the Whin-Sill (in the bed of the Lune,) where its 
whole thickness is not more than eleven or twelve feet, it has 
lost its well defined texture, and very nearly resembles the dykes 
described in my preceding Paper (supra, p. 21.) The same ob- 
servation applies to the other masses of trap in Lunedale, and to 
the mass on the north bank of the Tees, opposite to the mouth 
of the Lune. The two dykes in Eglestone burn, seem not to 
differ in any essential respect from the dykes of the Durham 
coal-field. 
From one of the last-mentioned localities, I obtained a spe- 
cimen which had a globular structure, and contained a great 
quantity of pyroxene, some of which was granular, semi-trans- 
parent, and of an olive colour. It was not, however, necessary 
to apply the flame of the blowpipe in order to separate these 
grains from olivine; because some crystals exhibited the ordinary 
characters of pyroxene gradually passing (probably from the effect 
of decomposition,) into a substance exactly resembling the olive- 
coloured grains above-mentioned*. 
* It is stated in my former Paper (supra, p. 34.) that parts of the decomposing dyke of 
Coatham 
Other varieties 
of Trap. 
