244 ME. C. T. TEECHMANN ON THE LITHOLOGY [June 1914, 



between these has been described by Dr. Woolacott as a plane 

 of thrusting. 1 Between here and Marsden the upper part is asso- 

 ciated with more or less definite ' breccia-gash ' phenomena. 2 The 

 gashes consist of fallen masses, both of segregated calcareous 

 fragments, and of fragments of the overlying concretionary and 

 bedded Upper Limestones mixed with dolomitic powder, collapsed 

 into fissures and cracks formed in the undulating surface of the 

 segregated and altered Middle Beds. 



Along most of the coast-section where this division is exposed, the 

 powdery dolomite has been washed away : consequently there has 

 ensued collapse and brecciation of the remaining calcareous material, 

 resulting in a bewildering series of unfossiliferous breccias which 

 flank the eastern side of the reef. These calcareous breccias become 

 in parts recemented, or have their interstices filled with superficial 

 matter washed in. In some cases, this material consists of powdery 

 dolomite from other parts of the rock, simulating a reverse type of 

 * breccia ' to that of the true cellular rock, where the enclosed 

 fragments are always more dolomitic than the cementing matrix. 



The thickness of these beds in the Marsden area is about 

 120 feet. No fossils, except plant-remains, are known from the 

 unaltered portions. 



Trow Rocks, near South Shields. — A patch of unchanged 

 rock, about 30 feet wide and 20 feet thick, surrounded and overlain 

 by hard, calcareous, cellular material which is quarried in great 

 masses for pier- works at the mouth of the Tyne. Fine lines of 

 incipient cellular structure are to be seen along some of the bedding- 

 planes of the unaltered rock, and the mass merges laterally with 

 every gradation into the adjacent hard, massive, cellular rock. The 

 whole rests upon well-bedded Lower Limestone (No. LIV), very 

 similar in composition to the unaltered Middle Beds. 



LI. Original, very thinly and evenly-bedded, porous rock, 

 enclosing flattened cavities, after a soluble constituent, along the 

 bedding-planes. Plant-remains occur. This rock is soft enough 

 to crush between the fingers. 



The next two examples were taken where the above-described bed 

 passes into the hard cellular rock. They consist of angular fragments 

 of all sizes, of very incoherent yellow and white powder, cemented 

 in a hard, grey, calcareous matrix. 



LII. The powder removed from the cavities. Practically all of 

 it passes through a sieve of 240 meshes per linear inch. 



1 ' On a Case of Thrust & Crush-Brecciation in the Magnesian Limestone 

 (County Durham) ' Mem. Univ. Durham Phil. Soc. No. 1, 1909 ; and 'The 

 Stratigraphy & Tectonics of the Permian of Durham (Northern Area) ' 

 Proc. Univ. Durham Phil. Soc. vol. iv, pt. 5, 1912. 



Horizontal pressures acting from without would doubtless have an 

 exaggerated effect upon such a rock as the Magnesian Limestone, weakened, 

 first by leaching-out of sulphates, and secondly by washing-away of powdery 

 dolomite. Such effects would not, in my opinion, be easy to distinguish from 

 the internal displacements of the formation incidental to these changes. 



2 G. A. L. Lebour, ' On the Breccia-Gashes of the Durham Coast, &c.' 

 Trans. N. Eng. Inst. Min. Eng. vol. xxxiii (1883) p. 165. 



