[Footnote 2: _Quart. Journ. Geol. Soc._ xl. (1884), p. 230.] It is especially worthy of remark that in the course of its nearly rectilinear course across the Durham Coal-field, 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.[3] Microscopic examination of this sill by Mr. Teall proved that the rock presents the closest resemblance to that of the Hett Dyke.[4] In this case, it may be regarded as probable that the two dykes and the intermediate sill form one related series of intrusions, and the conjecture that the Hett Dyke may be connected with the Whin Sill thus receives corroboration. The age of the Whin Sill itself will be discussed a few pages further on. [Footnote 3: Sir Lowthian Bell, _Proc. Roy. Soc._ xxiii. (1875), p. 544.] [Footnote 4: _Quart. Journ. Geol. Soc._ xl. (1884), p. 230.] 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 nearly 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,[5] and may perhaps belong to the same period of intrusion. [Footnote 5: Mr. Teall, _op. cit._ p. 244. _Quart. Journ. Geol. Soc._ xxxix. (1884), p. 656, and _Proc. Geol. Assoc._ (1886). See also Prof. Lebour, _Geology of Northumberland and Durham_, chap. xi.] The Great Whin Sill.--The geologist who, after making himself acquainted 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. The sea-cliffs of Bamborough and Dunstanborough, the rocky islets of Farne, 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. But not until he has traced these ridges for many miles southwards and found their component rocks to form there an almost continuous sheet does he realize that nothing of the kind among the Scottish Carboniferous rocks can be compared for extent to this display in the North of England.[6]