2 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
The age of this dyke cannot at present he satisfactorily fixed. It must 
be later than the Coal-measures through which it rises. Sedgwick long ago 
pointed out that though it reaches the escarpment of the Magnesian Lime- 
stone, it does not cut it; yet it is found in coal -mining to traverse the 
Coal - measures underlying the Limestone. He was accordingly inclined 
to believe it to be of older date than the Magnesian Limestone. At its 
western extremity it approaches close to the Great Whin Sill of Teesdale, 
though no absolute connection between the two has been established. Mr. 
Teall, however, has called attention to the similarity between the micro- 
scopic structure of the rock forming the Hett Dyke and that of the mass of 
the Whin Sill, and he is strongly inclined to regard them as belonging to 
the same period of intrusion. 1 
It is especially worthy of remark that in the course of its nearly 
rectilinear course across the Durham Coalfield, the Hett Dyke, where it 
crosses the Wear, is flanked on the north at a distance of a little more than 
two miles by a second parallel dyke of nearly identical composition. 
Between the two dykes, during mining operations, a sill about 20 feet thick 
has been met with, lying between two well-known coal-seams at a depth of 
about 60 fathoms from the surface, and extending over an area of at least 
15 acres. 2 Microscopic examination of this sill by Mr. Teall proved that 
the rock presents the closest resemblance to that of the Hett Dyke. 3 In 
this case, it may be regarded as probable that the two dykes and the inter- 
mediate sill form one related series of intrusions, and the conjecture that the 
Hett Dyke may be coimected with the Whin Sill thus receives corroboration. 
The age of the Whin Sill itself will be discussed a few pages further on. 
Of the other dykes which may possibly be coeval with the Hett Dyke 
we may specially note those which follow the same W.S.W. trend, for that 
strike differs from the general W.N.W. direction of most of the dykes. 
Two conspicuous examples of the south-westerly trend may be seen, one 
near Morpeth, the other north of Bellingham. The former dyke, as regards 
microscopic structure, is more neai'ly related to the majority of the series 
in the North of England. But that north of Bellingham (High Green) 
presents affinities both in structure and composition with the Hett Dyke, 4 
and may perhaps belong to the same period of intrusion. 
The Great Whin Sill.— The geologist who, after making himself ac- 
quainted with the abundant sills among the Carboniferous rocks in the 
centre of Scotland, finds his way into Northumberland, meets there with 
geological features that have become familiar to him further north. Tire 
sea-cliffs of Bamborough and Dunstanborougli, the rocky islets of Fame, the 
long lines of brown crag and green slope that strike inland through the 
Kyloe Hills and wind across the cultivated lowlands and the moorlands 
beyond, remind him at every turn of the scenery in the basin of the Forth. 
1 Quart. Joum. Geol. Soc. xl. (1884), p. 230. 
3 Sir Lowthiau Bell, Proc. Roy. Soc. xxiii. (1875), p. 544. » 
3 Quart. Joum. Gcol. Soc. xl. (1884), p. 230. 
4 Mr. Teall, op. cit. p. 244. Quart. Joum. Geol. Soc. xxxix. (1884), p. 656, and Proc. Geol. 
Assoc. (1886). See also Prof. Lebour, Geology of Northumberland and Durham, chap. xi. 
