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



of the breccias or from the calcite of the segregated rocks or from 

 the powdery dolomite in other parts of the limestone. 



3. The Magnesian Limestone has seldom been deformed without 

 fracture, so that flow-structures 1 are not produced in it on a large 

 scale, but at one or two places shear and flow structures and strain- 

 slip planes are developed, and at Claxheugh the Yellow Sands were 

 sheared up for several feet over a mass of breccia. At Hendon, along 

 the thrust-plane, incipient flow-structures (a kind of megascopic 

 pseudo-stromatic or phacoidal structure) has been developed in it. 



4. At Claxheugh shear-planes on a large scale can be seen in the 

 reef, the rock having given under the horizontal pressures into 

 large lenticular masses. 2 



5. Folding due to thrusting is developed on a broad scale and on 

 a very minute one. The broad syncline of the rigid Upper 

 Magnesian Limestone beneath Sunderland, which extends from 

 Marsden to Hendon, and is 6 miles across, was produced by these 

 horizontally directed pressures, 3 and the dome in Cullercoats Bay is 

 also due to them. Folding on a smaller scale is common at Marsden, 

 and in one part of this section overfoldingcan be seen. At Claxheugh 

 and in Cullercoats Bay overfolding on a very minute scale occurs in 

 the softer beds of the Marl Slate. 



6. Compression-jointing and fracture-cleavage are developed in the 

 Magnesian Limestone at Marsden and slightly in other areas. At 

 Marsden the distance between the parallel partings varies from a few 

 inches to a minute fraction of an inch, the rock fracturing along these 

 close parallel planes and the structure being seen for over a quarter of 

 a mile along the coast (see Memoir on the Marsden area, section, 

 fig. 8). In my paper on that section I have referred to this 

 fissility of the rock as cleavage, a term which has been objected to 

 by one geologist, who obviously had not seen the Marsden section, as 

 inapplicable in connexion with these rocks. In that paper I defined 

 the term clearly as "the flattening of the mineral particles by 

 pressure, so that they have a parallel arrangement with their shorter 

 axes in the direction of greatest stress ". 4 As the word cleavage is 

 now used in connexion with rocks in two senses — " fracture-cleavage " 

 and "flow-cleavage" 5 — I gave this definition in order to show 

 clearly that 1 was using the term simply to denote a compression 

 structure, i.e. fracture-cleavage, close- joint-cleavage, etc., and not 

 flow-cleavage. I showed that this compression-jointing (when coarse 

 it may be called this) or fracture-cleavage was developed at Marsden 

 at right angles to the pressure after the folding, but before the 

 brecciation of the rock took place. 



7. Crush breccias, where beds have been thrust against a horst-like 



1 i.e. permanent change of form without conspicuous fracture. 



2 See section of Claxheugh in Trans. Nat. Hist. Soc, op. jam cit., 1919. 



3 A general section of the coast is given in my paper on North Durham, fig. 9. 



4 Memoir on Marsden, p. 10. Sorby showed that this structure had been 

 impressed on Devonian Limestones by pressure ' ' On Slaty Cleavage as 

 exhibited in the Devonian Limestone of Devonshire ' ' : Phil. Mag. , vol. xi, p. 20 ; 

 vol. xii, p. 127, 1856. 



5 Leith, Structural Geology, p. 10. 



