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



in tuberculation, while Conulus is tending towards an increase in 

 elaboration. The extremely rapid introduction of new tubercles in 

 C. albogalerus, the latest species considered, suggests that any 

 descendants of that form would have such complex and multiple 

 tuberculation that the original " Holectypoid" quality of recognizable 

 order in the pattern would disappear. 



The results of this, and of the earlier, part of the paper may be 

 tersely summarized as follows : Pyrina gave rise to Conulus early in 

 the Cretaceous period. Later, in the Tertiary, Pyrina became 

 modified into Eehmoneus. The Pyrina— Echinoneus stock was almost 

 static in evolution; but Conulus, with changed habitat, diverged 

 rapidly along a new path which must have led away from the 

 Holectypoida. Both Pyrina and Echinoneus must be classed with the 

 Holectypoida. 



EXPLANATION OF TEXT-PIGUEE, p. 445. 

 All the figures in each horizontal line (i.e. representing a single species) are 

 drawn to the- same magnification ; but the different species are variously 

 magnified to aid comparison. The median tubercles on each plate are shaded. 

 Apart from the dotted lines linking the elements of the chevrons, there is 

 nothing diagrammatic in the figures (unless the omission of all smaller 

 ornament be so regarded). Tubercles without shading or linking lines are 

 either supernumeraries or members of chevron patterns whose associates are 

 undeveloped. 



III. — The Magnesian Limkstone of Durham. 



By David Woolacott, D.Sc, F.G.S. 



Part I. 



Conditions of Deposition — Former Presence of Sulphates — Formation of 

 " Demagnesified " Kocks — Development of Concretionary and Segregated 

 Structures, 1 etc. 



IN my paper on "The Stratigraphy and Tectonics of the Permian 

 of Durham (northern area)" 2 I endeavoured to give a descrip- 

 tion of the stratigraphical and structural features of these rocks with 

 as little discussion of the theoretical questions involved as possible. 

 I regarded a thorough examination of the strata in the field as the 

 only way in which to obtain a knowledge of the divisions of this 

 system, and as essential for elucidating the structural, physical, and 

 chemical changes that have taken place in the Magnesian Limestone. 

 It was my intention to describe similarly the southern area, and 

 afterwards to discuss theoretical and general questions relating to the 

 Durham Permian. Although a great part of the southern district 

 has been surveyed by me, I have decided to leave the description of 

 that area and give in this paper a general summary of recent work on 

 the Magnesian Limestone of Durham, with a discussion of the 

 theoretical views that maybe advanced to explain the observed facts, 



1 In Part I of this paper a general acquaintance with the divisions of the 

 Magnesian Limestone and with certain structural features of this rock is pre- 

 supposed ; readers not having such preliminary knowledge will find these 

 subjects summarized and discussed in Part II. 



2 Proc. Univ. Durham Phil. Soc, vol. iv, pt. v, 1911-12. 



