TRA?fSACTIONS OF SECTION C. 869 



G. On Arborescent Carhoniferous Limestone from near Bristol. 

 By Horace B. Woodward, F.R.S. 



A specimen of Carboniferous Limestone, showing arborescent markings, was 

 obtained by Mr. Spencer G. Perceval from Brentry Hill, near Henbury, Bristol, _ 

 and was presented by him in 1897 to the Museum of Practical Geology. /The 

 rock is about 6 inches thick, and the lower half is a current-bedded oolitic lime- 

 stone. The upper half comprises banded calcareous mud with a few layers of 

 oolitic grains, and the material in this portion of the rock has been disturbed, the 

 layers having been bent; while the hollows between the curves are partially 

 eroded and filled with irregular detrital material containing oolitic grains. JL-> 



The surface of the block presents an irregular concretionary structure, resem- 

 bling that seen on many varieties of Gotham Marble ; but it is not so pronounced 

 as in some of the mammillated surfaces seen in that rock. - , 



The appearances are probably due to mechanical disarrangement of the upper 

 layers produced prior to and during the consolidation of the rock, and they suggest 

 a pause in the deposition of sediment. --' 



It is noteworthy that the darker bands which produce the arborescent mark- 

 ings stand out slightly in relief on the weathered face of the block of Carboniferous 

 Limestone. This is also the case with an example of Gotham Marble which I 

 lately obtained on the South Wales Direct Railway at Stoke Gifford. 



A small specimen of Carboniferous Limestone from Backwell, near Nailsea, 

 given to me by Mr. W. H. Wickes, shows indications of arborescent markings. 

 [Further references to the subject are given in the ' Geol. Mag.,' Dec. 3, vol. ix., 

 1892, p. 110; see also B. Thompson, 'Quart. Journ. Geol. Soc.,' vol. 1., 1894, 

 p. 393.] 



7. Repoi't on Pliotographs of Geological Interest in Britain. 

 See Reports, p. 530. 



8. Jteport on Photographs of Geological Interest in Canada. 

 See Reports, p. 546. 



FRIDAY, SEPTEMBER 9. 



The following Papers and Reports were read : — 



1. The Comparative Value of Different Kinds of Fossils in Determining 

 Geological Age. By Professor O. C. Marsh. 



More than twenty years ago my attention was called to the subject of the 

 difference between the value of fossil Plants, Invertebrates, and Vertebrates, as 

 evidence of the geological age of the strata in which they were preserved. On the 

 comparative value of these different groups of fossils then depended the solution of 

 some grave problems in the geology of the Rocky Mountains. I therefore began a 

 systematic investigation of this subject, and gave the results in an address before 

 the American Association for the Advancement of Science in 1877.' I stated the 

 case as follows : — 



' The boundary line between the Cretaceous and Tertiary in the region of the 

 Rocky Mountains has been much in dispute during the last few years, mainly in 

 consequence of the uncertain geological bearings of the fossil plants found near 

 this horizon. The accompanying invertebrat*j fossils have thrown little light on 

 the question, which is essentially whether the great Lignite series of the West is 



• American Journal of Science, vol. xiv. pp. 338-378, November, 1877. 



