230 Scientific Proceedings, Royal Dublin Society. 



that there is a distinct tendency towards a quadrilateral form in the isolated 

 spots, and that' some of the more irregular are aggregates of quadrilaterals. 

 Onljr two or three were seen with a pentagonal shape, and no other outlines were 

 observed definite enough to be given a name. 



In a number of cases there are notches in the corners of quadrilaterals, with 

 linear sides approximately parallel to the sides of the quadrilaterals. 



On the same bedding plane are a specimen of Oldhamia antiqua, the cast of 

 an arched worm burrow about 3 mm. in diameter, another horizontal one about 

 03 mm. in diameter, and a shallow funnel-shaped depression surrounded by a 

 tumid elevation raised about 05 mm. above the general level, and 1 mm. above 

 the bottom of the depression. 



Horizontal and transverse sections were made through the spots. The latter 

 (PL XII, figs. 4, 5) showed that tlie spots are tabular bodies, with a thickness 

 varying little fi'om 03 mm. The rock is very fine grained, with well-marked 

 bedding, certain levels being marked by dark lines. The bodies in question lie 

 upon one of the bedding planes so marked, and have their upper surfaces on the 

 plane along which the slab was split. 



The light green shale is apparently composed chiefly of flakes of sericite 

 and chlorite and minute grains of quartz. The dark bands are due to an 

 increase in chlorite. The bodies we are studying, when examined with the 

 microscope in thin section, are seen to differ from the rest of the rock in that 

 the flakes of sericite and chlorite are, on the whole, larger, the chlorite more 

 abundant, and interstitial quartz more evident. 



A horizontal section of a reticulate specimen shows a marginal accumulation 

 of clilorite and an increase of colourless constituents forming the spots. In 

 some of the transverse sections thei'e is a light-coloured interior, with, apparently, 

 much quartz, a chloritic layer bounding it above and below, as well as at the 

 sides (PI. XII, fig. 4 left, and fig. 5 right). 



With regard to the origin of these curious markings, it is not easy to suggest 

 a satisfactory explanation. It seems very unlikely that they are of organic origin. 

 The possibility of their being crushed specimens of primitive cystids was 

 considered. The tabular form, the angular outlines, and the apparent presence of 

 a thick middle layer liaAang a less firm structure than the rest, are points of 

 resemblance. But the general quadrilateral, instead of pentagonal or hexagonal 

 shape, the angular notches, and the complete absence of any characteristic 

 structures, sueh as pores or marginal folds, seem to rule out this suggestion. 

 The angular nature of the markings appears incompatible with the activities of 

 worms. 



Turning to inorganic agencies, concretionary action could not produce such 

 sharp angles and straight edges. An inspection of some of the more regular 

 outlines, with their occasional notches and "outworks" (PL XII, fig. 6), 

 strongly suggests crystals. Fig. 7, PL XII, shows camera lucida drawings of 

 th« shapes of pseudomorphs of halite ciystals in Triassic marl. Some of the 

 shapes are remarkably like those we are considering. 



It is suggested, therefore, that the spots originated as crystals embedded in 

 the sediment. These subsequently became dissolved away, and the cavities 

 filled by mud. If the crystals had been those of an iron-bearing mineral, the 

 pseudomori^hs might have contained a residue of iron, especially along their 

 boundaries, which would help to account for the greater abundance of chlorite 

 in the spots, and its segregation at the edges in some of them. 



It seems unnecessary to assume that the original crystals were tabular, since 

 pseudomorphs in mud of equidimensional crystals would presumably be 

 squeezed flat by the compression of the sediment into shale. This would also 



