VI. 
The Internal Structure of this magnificent district, may, for the 
purpose of this essay, be thus briefly noticed. 
1. The Slaty Tracts. — Hougill Fells, the largest of all, consists of 
argillaceous rocks, the lowest in the series of Yorkshire strata. 
A small tract of nearly contemporaneous rocks, ranging eastward 
from Kirby Lonsdale to beyond Horton-in-Ribblesdale, yields valuable 
slate and lies under limestone. 
2. The Basaltic Tract. — This is an important feature in Teesdale ; 
from the sources of the river Tees to its junction with the Yorkshire 
Lune, there is little variety in the rock. Its effect on the adjacent 
limestone is remarkable, for it has converted the dark calcareous rocks 
to a bluish white and granular marble, or loose. ‘ sugar limestone.’ 
3. The Cave Limestone Tract. — By this title attention is directed 
specially to the wide surfaces and clifl’s of limestone, almost entirely 
unmixed with other substances, which lie at the base of Wharnside, 
Ingleborough, and Pennigent, and occupy a broad area about the source 
of the Aire. 
4. The Tract of Alternating Limestone, Sandstone, and 
Shale,' (with some Bad Coal), (Yoredale Rocks of Phillips) occupies 
a great breadth of surface, filling great part of Teesdale, Swaledale, 
Yoredale, and Wharfedale, and rising to a considerable height in the 
intervening ridges, hut seldom reaching their summits. A remarkable 
feature of geography in this tract, is a row of swallow holes in the line 
where each limestone bed comes to the surface. 
5. The Millstone Grit Tract. — This includes a series of coarse 
gritstones, finer flagstones, shales, ironstone and bad coal. The loftiest 
points (except Mickle Fell,) are formed of parts of this series ; it covers 
enormous surfaces between the dales, and occupies nearly all the eastern 
part of the north-western region. 
Aspect of Vegetation. — Throughout the north-western district, 
distinctions appear between the vegetable coverings of the slaty, basaltic, 
calcareous, shaly, and gritstone tracts, and sometimes they are obvious 
and even striking. Wherever the gritstone rocks rise to high ground 
they are thickly covered by heath, and often wrapped in deep and ancient 
peat ; beneath a craggy summit of such grit, runs a bluish green herbage 
of sedg6s, rushes, and grass, on a slope of argillaceous shale; and very 
often beneath or amidst these contrasted tints are bands of beautiful 
