240 
existence. They may be the kernels of an amygdaloid, due 
to alterations ; or some of them may be true pebbles. 
The next section in going down the river is at the Old 
Pencil Mill, below Cronkley. Cronkley, too, like Falcon 
Glints, is composed of Whin ; and beneath the Whin Crag can 
be seen in various places, particularly at the White Force 
(which probably takes its name from the conspicuous white 
Limestone there seen), a white sugary crystalline Limestone ; 
but the beds below the Limestone are not seen ; at least we do 
not see as far down as the Breccia. The hill side is covered 
with debris all the way down to the Pencil Mill. Here we 
have a very interesting section : beds of hardened Shale 
traversed by dykes of Mica trap. These dykes were di^:- 
covered by my friend, Mr. W. Gunn, who remarked their total 
dissimilarity to anything else in the country. They are, how- 
ever, like dykes in the Kendal country, which are not known to 
pierce the Carboniferous beds, but seem to be confined to the 
Silurians. The beds traversed by these d3^kes on the banks 
of Tees looked to me a deal more like Skiddaw slate than Car- 
boniferous Shale ; and they are crossed by divisional planes 
(whether bedding or cleavage) at high angles, while the 
Carboniferous beds are lying at low angle, and (it is needless 
to say) are not cleaved. Putting these three things together, 
viz., the Silurian character of the Mica trap dykes, the un- 
carboniferous look of the pencil beds (and I may here remark 
that in Westmoreland the Skiddaw Slate was once similarly 
worked for slate pencils, before such could be got more- 
cheaply from elsewhere), and the presence of Breccia at the 
foot of Falcon Clints, it struck me that we might have in 
the Breccia the base of the Carboniferous rocks reposing on 
Silurian rocks, possibly Skiddaw slate, but at all events 
Silurians; and I made the suggestion then and there to 
Messrs. Gunn and Clough, who accompanied me on my visit 
to the pencil beds. 
