Vol. 67.] AND TECTOXIC3 OP THE PERMIAN OF DURHAM. 313 



has been rendered crystalline, as well as more calcareous, and the 

 fossils have been obliterated. They do not afford any proof that 

 the rock has been dolomitized subsequent to deposition. The per- 

 centage of calcium carbonate is sometimes over 99, while that of 

 magnesium carbonate is occasionally as much as 50. 



The fauna of the Magnesian Limestone is very restricted (about 

 140 species) and most peculiarly distributed. The marked 

 palaeontological features d.re the profusion of individuals in the 

 Middle Fossiliferous Limestone (which appears to have formed 

 a shell-bank in the Middle Magnesian-Limestone sea) and their 

 sudden disappearance in the Upper Limestone. ~No corals, 

 echinoderms, polyzoa, brachiopods, or cephalopods have ever 

 been found above the top of the Middle Fossiliferous division : 

 only a few fishes, gastropods, lamellibranchs, entomostraca, and 

 foraminifera occurring in the Upper beds. The Lower and Middle 

 Fossiliferous Limestones are marked by the presence of Productus 

 horridas Sow. Fish-remains occur at two horizons : namely, the 

 Marl Slate and the Flexible Limestone, and the beds above these 

 deposits. 



The Brecciated Beds, which occur at various horizons, chiefly 

 however in the two Middle divisions, constitute the most marked 

 tectonic feature of the Magnesian Limestone of the area. They have 

 been produced by thrusting, which brought about a decrease in the 

 lateral extension of the Permian. Associated with the breccias 

 are other proofs of thrusting : (1) Thrust- or shear-planes ; (2) dis- 

 turbed and displaced masses of Lower Limestone; (3) intruded 

 breccias ; (4) slickensided and grooved, horizontal and vertical 

 surfaces ; (5) cleavage ; (6) folding, both on a local and on a general 

 scale; (7) buckling, thickening, and squeezing-out of beds ; (8) pha- 

 coidal and other structures ; and (9) Assuring. The main thrust 

 at Marsden appears to have acted from a few degrees south of east 

 to a few degrees north of west ; there are, however, distinct evi- 

 dences of movement from other directions in different parts of the 

 district. Experiments, made on the compressive strength of the 

 rocks affected by the thrust at Marsden, indicate that the thrusting 

 reached a maximum of about 300 tons per square foot. Observations 

 made by Mr. S. R. Haselhurst, M.Sc, in the Cullercoats area seem 

 to prove that the thrusting occurred later than the post-Permian 

 movement of the Ninety-Fathom Dyke — some faulting in the area 

 is, however, later than the thrusting — , and it appears evident that 

 the shattering of the strata was produced prior to the pre-Glacial 

 era of denudation. It may have been connected with the Miocene 

 movements that produced such marked changes in the physiography 

 of Britain. 



Discussion. 



The Chairman (Dr. C. W. Andrews) suggested that perhaps 

 comparison of the Permian dolomitic limestones with those of the 

 post-Miocene raised reefs of Christmas Island (Indian Ocean) might 

 help to throw some light on the question of dolomitization. 



Q. J. G. S. No. 267. z 



