^O.W.Card—FlexiUmyofRochs. 117 



-Northampton iron ore,^ and the pisolitic iron ore of Cader Idris." It 

 •diflfers from all these, however, in the fact that the grains have a 

 coating of carbon, and in the absence of the oxides of iron which 

 form the chief constituent of the latter. It resembles all the above- 

 mentioned rocks in the fact that the grains contain a skeleton of silica, 

 and further in the presence of ferrous carbonate and phosphoric acid. 

 'It is most probable too that the silica skeleton in the case of the 

 Hirnant limestone is due to the infilling of a cavity produced by- 

 solution of the calcium carbonate of the grains, which solvent action 

 :was prevented from extending inwards by the insoluble nature of the 

 carbon. On the whole it seems that the rock owes its distinctive 

 characteristics to vegetable agencjs — the peculiar form of the carbon 

 as well as the other chemical constituents lending evidence in sup- 

 port of this view. This evidence would be much strengthened if the 

 sandstone residues described above should turn out to be a result of 

 the existence of plant life as suggested. 



IV. — Ok the Flexibility op Eooks ; with Speoial Eeference to 

 THE Flexible Limestone of Durham. 



By George W. Card, A.R.S.M. ; 

 Assistant Demonstrator of Geology at the Eoyal College of Science, London. 

 TI^HE existence of rocks possessing, when the laminee are not too 

 , X thick, the property of flexibility has long been known. Upon 

 flexible sandstone (" Itacolumite ") a great deal has been written at 

 different times, and of late years important work has been done 

 which renders it necessary to greatly modify the opinions formerly 

 held with regard to this rock. Notwithstanding the interest which 

 is attached to the subject, it is one very much neglected by our text- 

 hooks, the British, either ignoring it altogether, or treating it with 

 the utmost brevity, the German, while sometimes referring to it at 

 considerable length, do not do more than enunciate the old views. 

 To Prof. Judd — who has kindly aided me with advice, and by afford- 

 ing facilities for preparing this paper — I owe the suggestion that it 

 would therefore be useful to give a resume of the present state of 

 our knowledge upon the flexibility of rocks in generaL This paper 

 will accordingly be divided as follows : — 



1. An account of the Durham Limestone. 



2. Some remarks upon Flexible Sandstone. 



3. A comparison of the two rocks. 



1. The Flexible Limestone of DurJiam. — My attention was first 

 'directed to the existence of this rock by Mr. H. B. Woodward's 

 " Geology of England and Wales." ^ Being well acquainted with 

 the Sunderland district, I determined to take the first opportunity of 

 looking for the rock, and, such an opportunity having occurred 

 during the past summer, I now give the results of my work. So 

 far as I am aware, there are only three references to this variety. 



1 Judd, Memoirs of Geol. Survey, Geology of Eutland, pp. 117-138; Hudleston, 

 Proc. Geol. Assoc, vol. xi. p. 104. 



2 Cole and Jennings, Q.J.G.S. vol. xlv. (1889) p. 426. ^ p. 219. 



