284 prof. w. j. sollas osr iCHNiuM [May. 1900, 



as wholly secondary. There may be isolated instances in which the 

 structure is accompanied by a difference in mineral composition,, 

 distinguishing it from the rest of the lamina, and these may be 

 original ; but in the great majority this is not the case ; and I 

 would venture to suggest that the structure, as typically displayed, 

 is the result of concretion, if we may use a term which is so- 

 often lamentably abused. I am the more inclined to this ex- 

 planation since I have been able to produce a closely similar 

 structure by artificial means. If a fragment of Oldhamia-slsite be 

 rubbed down on a hone, and the fine mud thus produced be allowed 

 to stand for a few minutes, a concentric arrangement of fine particles, 

 very similar to that already described, may be observed ; by pouring 

 the thin mud or muddy water on to a glass-slide the structure 

 develops under conditions which allow it to be studied by trans- 

 mitted light under the microscope, and it may then be photographed. 

 The cause of this phenomenon is not clear to me, but it is possibly 

 connected with cohesion ; whatever the cause, the concentric arrange- 

 ment in the OWiamia-beds appears to be a similar effect, while the 

 associated linear streaks of opaque granules may have resulted from 

 the slow movements of water as it escaped from the gradually 

 consolidating sediment of which the beds consist. 



The nature of Oldhamia cannot yet be regarded as definitely 

 ascertained. That it is of organic origin seems to me scarcely to 

 admit of doubt ; whether the organism were animal or plant, and 

 in what manner it imprinted its traces on the rocks are questions 

 that still await a definite solution. 



Ichnium Wattsii, gen. et sp. nov. (PI. XVII, fig. 1.) 



A radiate system of continuous, undulating, bifurcating furrows, 

 of a uniform width of 0'5 to 0'6 mm., of a maximum length of 

 28 mm., with rounded terminations. Central area occupied by 

 cylindrical elevations, which plunge downwards into the matrix- 

 Occurrence : indurated red shale, \ mile south of Bray Head. 



Bibliography. 



(1) Oldham, Thomas. ' On the Rocks at Bray Head ' Journ. Geol. Soc. Dublin y 



vol. iii, p. 60. 1844. 



(2) Foebes, E. ' On Oldhamia, a new Genus of Silurian Fossils ' Journ. GeoL 



Soc. Dublin, vol. iv, p. 20. 1848. 



(3) Kinahan, J. R. ' On the Organic Relations of the Cambrian Rocks of Bray 



& Howth ; with Notices of the most Remarkable Fossils ' op. cit. vol. viii, 

 p. 68. 1852. 



(4) . ' The Genus Oldhamia, its Characters, etc' Trans. Roy. Irish Acad. 



vol. xxiii, p. 547. 1858. 



(5) Gceppeet, H. R. 'Flora der Sil. Dev. und Unter-Kohl. Formation' Nova 



Acta Acad. Ca>s. Leop.-Carol. Naturae Curios, vol. xxvii, pp. 437, 441, & 

 pi. xxxiv, figs. 1 & 2, pi. xxxv, fig. 1. 1860. 



(6) Baily, W. H. Geol. Mag. vol. ii, pp. 385, 391, with figures. 1865. 



(7) Etheeidge, R., & Saltee, J. W. Mem. Geol. Surv. (Geology of North 



Wales) vol. iii, p. 281. 1866. 



(8) Schimpee, W. P. Traite de Pal. Veget. vol. i, p. 182 & pi. ii, fig. 10. 1869. 



(9) Bailt, W. H. ' PaUeontological Notes accompanying Explanations to 



Sheets 121 & 130 of the Maps of the Geological Survey of Ireland ' Mem. 

 Geol. Surv. Irel. 1869, pp. 17-19 k & figs. 1-3. 



