352 marker: igneous dykes of the north of England. 



of the well-known Whin Sill, a great sheet of diabase extending 

 from the Eden Valley to the Fame Islands ; and there is little 

 doubt that some of the Northumberland and Durham dykes, such 

 as that of Hett, belong to the same date. This, however, raises a 

 difficult problem — the separation of these dykes from those of 

 Tertiary age. There are in the North of England a large number 

 of dykes of basic and sub-basic characters, cutting Carboniferous 

 rocks, but — with one or two exceptions — not entering the Mesozoic 

 tracts. The difficulty is to separate those injected in the interval 

 between the Carboniferous and Permian from others which, there 

 are good reasons to beHeve, are of Lower Tertiary age. Of course, 

 if a dyke can be seen to rise through the uppermost Carboniferous 

 beds and be abruptly cut off at the base of the overlying strata, 

 there can be no doubt of its pre-Permian age ; but such relations 

 can rarely be verified. The only large dyke which cuts through 

 Mesozoic rocks is that of Cleveland, which intersects the Eower 

 Oolites at its eastern extremity, and this may be referred, without 

 hesitation, to the only subsequent period which we know to have 

 produced igneous rocks in Britain — the Lower Tertiary. Unfor- 

 tunately, the majority of the dykes in question occur only in tracts 

 occupied by Carboniferous and older strata, and cannot be traced to 

 the edge of the Permians, thus leaving it a matter of doubt to which 

 of the two groups any individual dyke is to be referred. According 

 to Mr. Teall, petrological study affords no certain data for separating 

 the earlier set from the later. Perhaps some conclusions might he 

 drawn from their bearings, if regarded collectively. Prof. Lebour 

 places the dykes of Northumberland and Durham in two classes — 

 (a) those striking E.-W. or E.S.E.-W.N. W., and (d) those strikmg 

 N.E.-S.W. or E. N.E.-W.S.W. The former class, which seems to 

 have the more uniform direction, probably includes dykes mostly of 

 Tertiary age, while pre-Permian dykes may perhaps be looked for 

 among those of the second category. The former are to be linked 

 with an enormous development of basic dykes in southern, central, 

 and western Scotland ; and they have recently derived a new interest 

 from Dr. A. Geikie's theory, which makes them the channels of great 

 fissure-eruptions of basaltic lava in early Tertiary times. Many of 

 these dykes, scattered from Whitby to Stornoway, are remarkable for 

 their singular persistence in nearly straight lines for very great 

 distances. For example, the great augite-andesite dyke — variously 

 known as the Cleveland, the Cockfield, and the Armathwaite dyke — 

 runs from near the coast of North Yorkshire, through Durham and 

 Cumberland, to near Carlisle, a length of i lo miles, and possibly to 

 near Ayr, eighty miles farther. Dr. Geikie has also drawn attention 



Nalurali.->t, 



