Br. D. Woolacott — Magnesian Limestone of Durham. 493 



The thrust-planes, folding, compression-structures, and other 

 evidence of horizontal movement in these rocks are quite distinct, 

 affect beds beneath which sulphates were laid down, and cannot 

 have been directly caused by the removal of beds of gypsum or 

 anhydrite, although a certain instability of the strata had most 

 probably been produced by solution of such beds before the thrusting 

 took place. The general effect of the thrusting has been to produce 

 a decrease in the lateral extension of the Magnesian Limestone ; the 

 rocks — Coal-measures and Permian — which have not been fractured 

 and brecciated have been bent into broad folds. The syncline of the 

 Permian strata beneath Sunderland appears to be the direct result of 

 these movements (see section, fig. 9, in my paper on North Durham, 

 op. jam cit.). 



The horizontality or general low hade of the thrust-planes, the 

 damping down of the folds, the shear and flow structures prove that 

 a considerable weight of rock lay above the thrust horizons at 

 present exposed at the surface, when the deformation of the area 

 took place. 



It will probably be useful to geologists and geological students if 

 I give a brief summary of the phenomena I have observed as being 

 directly due to thrusting, with a short discussion of one or two 

 points to which my attention has been drawn since I first wrote my 

 account of this district. 1 They may be summed up as follows : — 



1. Major and minor thrust-planes, with slickensided surfaces both 

 above and beneath them. These planes occur on certain horizons, 

 the most continuous being the top of the Lower Limestone. The 

 top layers of this limestone have acted as a base on which the more 

 cellular cavernous rocks of the Middle Limestone have been moved. 

 The thrust-planes are not, however, confined to this horizon : some- 

 times minor fractures cut down through the Lower Limestone and 

 affect the Marl Slate and Yellow Sands beneath, so that large 

 masses of the Lower Limestone are displaced. These planes do not 

 pass into the Yellow Sands, which as a rule appear to have given 

 freely to the movement. In the railway cutting at Claxheugh 

 highly disturbed beds of Lower Limestone are thrust over beds of 

 the reef. Thrust-planes, however, occur at other horizons than at 

 the junction between the Lower and Middle Limestones, the most 

 marked being exposed in the cliff at Hendon at the southern limit of 

 the broad syncline of the Concretionary Limestone beneath Sunderland. 

 At this place the Middle Limestone is thrust over the Flexible and 

 lower part of the Upper Limestone. 2 



2. Mylonized rock occurs along the thrust-planes. It is best 

 developed at the Trow Hocks and in Frenchman's Bay. This 

 peculiar layer of rock, an inch or two in thickness, which runs along 

 the top of the disturbed beds or along the thrust-planes, is quite 

 distinct in colour, appearance, and texture from the cementing material 



1 Fuller evidence is detailed in my memoir on the Marsden district, my 

 general paper on the Permian of North Durham, and the paper in the Proc. 

 Geol. Assoc. Photographs of many of the structures described will be 

 found in these. 



2 See section, fig. 10, in Permian of North Durham. 



