Composition of 
the Trap. 
164 Professor SEDGWICK on the 
coated over with innumerable lichens, and their surface is some- 
times obscured by a brown ochreous incrustation. Where they 
are best exposed they appear, according to the circumstances 
of their position, of various shades of colour between light-grey 
and dark-brown. From the other rocks of the district they are 
easily distinguished, even at considerable distances, by the harsh 
and angular appearance of their escarpments. This phenomenon 
arises, partly from the prismatic structure of the trap, partly 
also from its indestructible nature. Throughout the district it 
seems admirably formed for resisting the decomposing powers of 
the elements; so that the blocks generally present their rude 
angular forms in great perfection. 
There is not a single remarkable escarpment of the trap in 
which we cannot discover traces of a prismatic structure, and in 
some places the vertical lengthened prisms give the rock a fine 
columnar aspect. The best examples of this structure are seen 
below Caldron Snout, in the bed of the river near Winch 
Bridge, and in some of the escarpments near the village of 
Holwick. In the arrangement and dimensions of the prisms there 
is not in general any approach to regularity; neither did I dis- 
cover any traces of those natural jomts which are so common in 
the columns of overlying basalt. 
A globular structure is very common in the beds of trap, 
associated with the mountain-limestone, in Derbyshire and some 
other parts of England; but in Teesdale there is not a single 
good example of that arrangement. It is, indeed, most frequent 
in rocks of the class we are now describing, which are in a state 
of disintegration, or are naturally of an earthy texture. But the 
trap of High Teesdale, with a very few exceptions, is of a highly 
crystalline texture, and perhaps on that account the globular 
form is less frequent. 
All the sound specimens are very sonorous, and offer great 
