282 PROF. W. J. SOLLAS ON IOHNIUM [May I9OO, 



themselves as straight parallel lines or bands, but when Oldhamia 

 is present the laminae are curved conformably with the grooves or 

 ridges of the impressions (PL XIX, fig. 15). In some cases there is 

 a maximum inflexion confined to a single lamina not more than 

 O'l mm. in thickness, and the curvature rapidly passes away above 

 and below, so that no signs of it remain at a distance of about 1 mm. ; 

 in other cases the maximum deflection is uniformly repeated by suc- 

 cessive layers through a distance of several millimetres. This fact is 

 very difficult to explain on any hypothesis, most of all on that which 

 invokes the agency of worms. It may be objected that the successive 

 conformable wrinkles of any one 0ldha7iiia-system represent a succes- 

 sion of single systems. But if this were so, two tangential sections at 

 an interval of some millimetres apart should not show an identical 

 ' pattern' (PI. XVIII, figs. 6 & 8); no burrowing animal would make 

 successive grooves vertically over one another of the same character 

 and in the same direction. Nor on the other hypothesis to be pre- 

 sently approached, that Oldhamia may be the trace of a plant, would 

 whorls of appendages be successively produced which resembled 

 one another in every particular, both of size, direction, and form. 

 There would appear to remain one, and only one, explanation : the 

 inflection of one lamina alone is original, that of the others is 

 secondary and the result of pressure. But the conditions requisite 

 to bring about this result involve the existence for a time of some 

 resisting substance in the place now indicated by Oldhamia. 



We might then represent matters to ourselves as follows : — Taking 

 first the case of 0. radiata, let us suppose it to represent, as Berkeley 

 acutely suggested, the branches of some calcareous Siphonaceous 

 alga ; its lower surface will form an impression on the surface of 

 the mud on which it must be supposed to rest ; if deposition be 

 slow, it will decay before many millimetres of sediment have 

 covered it, and a single groove will be all that remains to mark its 

 place ; but if deposition on the contrary be rapid, it may be buried 

 beneath a considerable thickness before it disappears, and the vacant 

 place that it leaves behind will be filled by a downward sinking of 

 the superincumbent laminae. It has for a long time seemed to me 

 that a fairly average rate of deposition would be about 300 mm. 

 per century, and if we assume that the Oldhamia-besLr'mg rocks 

 sometimes accumulated at the rate of 500 mm., then two years 

 would be required for the deposition of 10 mm. If Oldhamia were 

 so long in passing away, the overlying laminae might adjust them- 

 selves to the resulting cavity, so as to repeat the form of its floor 

 through a thickness of 3 mm. 



Turning next to O. antiqua, we should have to assume that 

 its upper surface was for some reason more resistant than the 

 lower, and that as it yielded from below the underlying sediments 

 rose up to take its place ; this species would thus be represented 

 by a ' creep ' of the laminae, the other by a ' sit.' That a very 

 slight difference in conditions determined whether a sit or a creep 

 should take place is indicated by the fact that sometimes one part 

 of O. antiqua exists in relief and another in depression. Although 



