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No. 27. 



ON A PROBLEMATIC STRUCTURE IN THE OLDHAMIA ROCKS OF 

 BRAY HEAD, CO. WICKLOW. 



By LOUIS B. SMYTH, M.A., Sc.B. 



(Plate XII.) 



(Read May 29. Printed July 6, 1923.) 



The rocks of Bray Head, Co. Wieklow, have long been well known as the original 

 source of the fossils Oldhamia antiqua and Oldliamia radiata. The only other 

 structures found in them, which may, with fair confidence, be regarded as due 

 to organisms, are wonn buiTows. Besides these, a small number of markings 

 have been claimed, but not generally accepted, as of organic origin. 



The series has generally been attributed to the Cambrian formation, but its 

 stratigraphical relations are somewhat obscure. A Pre-Cambrian, and even an 

 Ordovician, age has been suggested. 



This being the state of affairs, it seemed that any new structure from these 

 rocks might prove of interest, and should be described. 



A piece of green shale bearing certain peculiar markings (PL XII, fig. 1-3) 

 was collected by Mr. T..'W. Moran from a loose block on the railway south of 

 Bray Head. The slab is split into two along the bedding, and, on the bedding 

 plane thus exposed, is a group of markings, first pointed out by Professor Joly. 

 These are darker green than the matrix, and rather inconspicuous. On wetting 

 the surface, however, they become clearly differentiated. Any doubt as to the 

 origin of the specimen from the Bray Head Series is set at rest by the presence, 

 on the same bedding plane, of Oldliamia antiqua (PI. XII, fig. 3). 



Some of the spots are of a imiform green colour, but others (see PL XII, 

 lower part of fig. 1) have a reticulate pattern, consisting of groups of yellow 

 patches, separated by dark green lines, which form an irregular network. The 

 yellow patches vary from light greenish yeUow to a rusty yellow. In the 

 latter case the patch is often depressed, as though its substance had crumbled 

 away, the dark green boundary remaining as a slightly elevated rim. 



The largest continuous patches, about four in number, are about 15 to 25 mm. 

 long, have a very irregular and ragged shape, with, for the most part, angular 

 outlines, and show the reticulate structure. Their appearance suggests that 

 either they are aggTcgates of the smaller elements to be described, or they are 

 disintegrating into such units. The smaller spots are mostly between 1 and 

 3 mm. in diameter, though there are some smaller and some larger. Many of 

 the smaller spots are plain green, but others show the reticulate structure more 

 or less clearly. 



The most striking point about the spots is that the majority have angTdar 

 outlines (PI. XII, fig. 6). In a great many eases they are bounded by lines 

 which are straight, or nearly so. Quite a number approximate to a parallelogram 

 or rectangle in shape. In fact, after a careful study one gets the impression 



SCIENT. PROC. R.D.S., VOL. XVII, NO. 27. 2 U 



