﻿2 MISS M. K. HESL0P AND DE. J. A. SMYTHE ON THE [Feb. I9IO, 



and Morpeth Dykes have been clearly described by Prof. Lebour. 1 

 All three belong to the series which in Northumberland has a 

 general east-and-west trend. The Crookdene Dyke also runs 

 east and west, and for this reason, and chiefly because of its strong 

 petrological resemblance to them, it must be classed with the dykes 

 just mentioned. 



I. The Crookdene Dyke. 



About 16 miles west of Morpeth, in latitude 55° 8' 35" N., longi- 

 tude 2° 2' 45" W., there occurs a small exposure of a basaltic dyke 

 in the bed and banks of the Wansbeck, which is there a very small 

 stream. Close by is the farm of Crookdene, a name which may-serve 

 to distinguish this dyke. The exposure is greatly obscured by talus ; 

 three patches of the rock can, however, be examined : one at the top 

 of a steep bank about 30 feet high, another about half way down 

 the bank, and a third at the foot of the bank and in the burn. The 

 dyke occurs in a fault running east and west, which, from a con- 

 sideration of the surrounding country, apparently throws about 

 25 feet to the south ; it is intruded in the rocks of the Bernician 

 Series of the Carboniferous System, which in that part consist 

 largely of thick limestones and sandstones varied with shales. The 

 direction of the dyke is athwart the strike of the Whin Sill, which 

 crops out a little to the west, and across a surface-break in this 

 formation about a mile wide. No connexion can thus be traced 

 between the two igneous rocks, and although not greatly dissimilar 

 to the naked eye and in chemical composition, 2 they are very different 

 in microscopic structure. 



The northern face of the dyke is fresh and unweathered, and in 

 close contact with limestone and shale. On the southern face 

 there is a broad breccia-tilled fissure 12 feet wide, and the rock in 

 contact with this is weathered into a brown sandy-looking rock 

 for a depth of about 9 inches, the fresh basalt being about 1 foot 

 9 inches thick. The fault-fissure is filled with a soft breccia of 

 sandstone, limestone, shale, and some fragments of much decomposed 

 basalt. It would be difficult to imagine the intrusion of molten 

 material through a broad breccia-filled fissure in such a way that 

 the resulting dyke clung closely to one wall of the fissure, retained 

 a uniform thickness, and sent no offshoots apparently into the soft 

 breccia. We would suggest, in explanation, that Assuring took 

 place subsequently to intrusion, and that the fragments of basalt in 

 the breccia were torn off the dyke during the Assuring. 



In the lowest (easternmost) exposure, in the burn, the dyke is 

 seen to stop short in a bed of limestone, and the contact is highly 

 metamorphosed, the basalt being converted into white trap. This 

 occurrence suggests that the dyke rises steeply from below, and 

 consequently that we are near the head of the dyke at its eastern 

 end. The fact that the dyke is traceable west, but not east, of the 

 burn-exposures lends some support to this view. This is a point of 



1 ' Outlines of the Geology of Northumberland & Durham,' 2nd ed. (1886) p. 86. 



2 See Dr. Teall's analysis of the Whin Sill from Borcoyicus, Quart. Journ. 

 Geol. Soc. vol. xl (1884) p. 654. 



