26 
limit of the middle zone. The lower zone includes the whole of the 
eastern portion of the district, the valley of the Tees till above its junc- 
tion with the Lune, and the lower levels of the subordinate dales. 
Structure. — The course of the Tees from the Caldron Snout to the High 
Force runs over a mass of basalt, called the Whin Sill, which ascends 
Maizebeck for about two miles, and attains an elevation of about 1800 feet, 
in the almost inaccessible cliffs of Cronkley. By contact with the 
plutonic rock, the narrow' ridge of scar limestone which borders it, the 
oldest in our series of deposits, is in a great measure metamorphosed into 
a coarsely granular “ sugar limestone,” wdiich produces several plants not 
found elsewhere in the riding. Next in succession, over Micklefell and 
along the western margin of the county, extend Yoredale limestones, which 
also cover a small district south of the junction of the Greta with the 
Tees. Whilst on the east, the broad moors which surround the dales of 
of the Balder and Greta, and extend to the summit of drainage on the 
south, consist of millstone grit. In the neighbourhood of Croft, are 
traces of Permian deposits, wdth their botanical concomitant in Anemone 
Pulsatilla : and from thence to the boundary of the district stretches the 
new red sandstone of the central vale. 
No. VIII.— WEST SWALE. 
Drainage. — The Swale takes its rise by numerous branches amongst 
the lofty series of undulated summits which form the boundary between 
Yorkshire and Westmoreland. From Hollow Mill-cross the main branch 
flows due east for about five miles, receiving numerous inconsiderable 
feeders both from the north and the south. Opposite the celebrated Auld 
Gang lead mines it curves towards the south, the valley widening to en- 
circle the curious conical mound of Keasdon, 1639 feet in altitude. South- 
ward of this, below the small town of Muker, it again advances due east, 
and maintains that direction till Sw r aledale loses itself in the great 
central vale. Margined by the scars of the gradually descending summits, 
at Reeth it receives the considerable stream which rises near Baxton Nab 
and drains Arkengarthdale. Below Grinton the dale widens and becomes 
more highly populated and cultivated. At Marske the river receives an 
addition from the moorlands on the north ; and after flowing past Rich- 
