492 Dr. D. Woolacott — Magnesian Limestone of Durham. 



displacements and disturbances caused by it can be seen in the Coal- 

 measures, Yellow Sands, Marl Slate, Lower, Middle, and Upper 

 Limestones. The remarkable feature about the thrusting is that it 

 is directed to a more or less central area to the south of Marsden. 

 The only exception recorded to this is the well-known case of thrust 

 and unconformity in the Coal-measures at Whitley. 1 At this place 

 Lebour and Smythe regard the thrusting as being directed towards 

 the north, the massive upper sandstone having moved over the 

 shales, etc., beneath it from south to north, but from my detailed 

 study of the district I have for a long time regarded the Whitley 

 section as a " lag fault", i.e. the under beds have moved south under 

 a southerly directed pressure, leaving the massive sandstone behind. 

 The result is the same, but this explanation of this section is 

 necessitated, as the force acting along the Northumberland and 

 Durham coast towards Marsden was a general southern one, and the 

 movement of the strata was in that direction. 2 In my paper on the 

 Permian of North Durham I give a map showing the direction of 

 the thrusting movements that have been recorded in the Coal- 

 measures and Permian of this area. 



The Magnesian Limestone of Durham offers exposures of some of 

 the structures which have been proved to exist in other regions 

 of more intense thrusting, in so far as they could be impressed on 

 these rocks by the magnitude of the pressures acting and on beds of 

 such little variation in mineral composition. It is noticeable that 

 in the strata affected by the thrusting flow-structures are not 

 extensively developed. 3 It is only in portions of the softer rocks 

 and along thrust-planes that incipient flow-structures occur, while 

 brecciation, fracturing, etc., are developed on an extensive scale. 

 These rocks are therefore of interest as giving an example of 

 thrust movements- affecting beds within the zone of fracture, 4 

 although locally parts of the rocks lay within their zone of flow. 

 I have endeavoured to prove that the magnitude of the pressures 

 acting in the Marsden area reached 300 tons per square foot. 5 



The marked effects produced by these horizontal movements in the 

 Permian — movements which do not appear to have been greater 

 than some 300 feet in displacement — are probably partly due to the 

 cavernous, cellular, and porous nature of the rock caused by the 

 removal of the sulphates which probably had been largely removed 

 before the thrusting took place, although I think the rocks contained 

 sulphate solutions at the time the pressures acted (Geol. Mag., 

 October, 1919, Pt. I, p. 459). 



1 Q.J.G.S., vol. lxii, p. 530, 1906. 



2 Haselhurst gives evidence proving the pressure was from the north at 

 Cullercoats, Univ. Durham Phil. Soc, vol. iv, pt. i, 1910-11, and I prove that 

 the movement at Marsden was from the north-east. 



3 Flow-structures occur in the Coal-measures at Whitley, in the limestone 

 above the thrust-plane at Hendon, in the Yellow Sands at Claxheugh, etc., but 

 never on a large scale. 



4 Van Hise, "Principles of North American Pre-Cambrian Geology": 

 Sixteenth Ann. Eep. U.S. Geol. Surv., p. 589, and Leith, Structural Geology, 

 p. 3. 



5 Memoir on Marsden area, op. jam cit., p. 5. 



