198 



0. Holtedahl — Structures like Walcott's 



on tlie other hand, give no very distinct idea of their 

 character. In 1891, Garwood published a paper entitled 

 ^ ^ On the origin and mode of formation of the concretions 

 in the magnesian limestones of Durham/ ^^ treating, 

 however, mainly concretions of the more ordinary types, 

 some of which are depicted. In the British Association 

 report for 1900, Abbott has an abstract of a paper on 

 these concretionary types, and two years later another 

 very short abstract.-^ Trechmann in 1914 writes on 

 ''The lithology and composition of Durham magnesian 

 limestones" and in an earlier paper has remarks on the 

 concretionary structures.'^ Other very important contri- 

 butions are Woolacott's papers of 1912 and 1919.' 



Fig. 4. — Four-fifths nat. size. Compare with Greysonia basaltica Walcott. 



The purpose of the present article is not to give a 

 detailed description of the Durham concretions, since 

 such may be found in the papers above mentioned, and 

 since a full description with necessary illustrations will, 

 it is to be hoped, soon appear from the English investi- 

 gators, but rather to draw attention to the resemblance 

 of some of these Permian concretionary structures to 

 those of the Algonkian Newland limestone, and to illus- 

 trate this likeness by photographs of specimens which 

 were collected during the excursion and kindly sent to 

 Kristiania by Mr. Hawkes. 



^ E. J. Garwood, Geol. Mag., dec. 3, vol. 8, 434-440. 



'G. Abbott, Eept. British Assoc. Adv. Sci., for 1900, pp. 737-739; Quart. 

 Jour. Geol. Soc, Loudon, 59, 51, 1903. 



'C. T. Trechmann, Quart. Jour. Geol. Soc, London, 70, 232-265, 1914; 

 Ibid., 69, 184-218, 1913. 



^D. Woolacott, Proc. Univ. Durham Phil. Soc, 4, 241, 1912; Geol. Mag., 

 dec 6, 6, 452, 1919. 



