Prof. W. Keeping — Geology of Aherystivyth. 535 



again around Pont Erwyd the beautiful Nereites Madeani is quite 

 abundant. All these show clearly the marvellously tortuous tracks 

 of marine worms; centrally we see the depression made by the 

 body, and at the sides are the marks of the numerous lateral " feet " 

 with their bristles. Nemertites and other supposed worms are also 

 found. 



Others amongst these markings are quite as evidently marine 

 plants — Algse — seaweeds. There is one well-marked type of these 

 that is particularly noteworthy, consisting of a stalk, sometimes 

 several inches long, from the summit of which branches are given 

 off in a fan-shaped manner. Of this type there are three species — 

 a large, a small, and a medium-sized form. There are also long 

 strap-like seaweeds. 



Many others of the queer irregular markings ^ so abundantly 

 found may have had their origin jorimarily in some organism, or 

 the trace of some organism,^ to serve as a nucleus around which 

 material has gathered by " concretionary action." We may to some 

 extent explain this concretionary action as a force which causes 

 particles of the same nature to collect together into masses ; thus 

 it was that the m^^riad flinty particles originally in the chalk were 

 dragged together to form flints, so the particles of sulphide of iron 

 came together to form large crystals or nests about Aberystwyth, 

 and so also many of the irregular masses in our local grits have 

 had their origin. 



" Cone in Cone " Nodules. — Amongst the best-marked of these 

 concretions are the lenticular nodules, which occur in zones, well 

 seen along the coast between Aberystwyth and Borth. They are 

 sometimes so close together as to unite with one another ; but good 

 specimens, with circular contours, about 2 in. to 10 in. in diameter, 

 are not difficult to find. The peculiar and pretty structure known 

 as "cone-in-cone" is invariably found over the exterior of these 

 nodules. We may liken this " cone-in-cone " structure to crowds 

 of miniature conical sugar-bags packed one into the other, and all 

 the packets arranged side by side around the stone, moreover every 

 sugar-bag has its sides delicately crimped. The bases of the cones 

 are directed externally, the apices inwards. Single cones may be 

 extracted with the knife, usually about a quarter, or half an inch 

 long, but larger ones are sometimes found. This is not an organism 

 — not a life-structure, and we must look to the results of crystalline 

 forces for some similar appearances to explain its origin. Now 

 ordinary calcspar is often found in a fibrous form, and I have seen 

 layers of such mineral where in some places the fibres run parallel 

 to one another, but in other spots they are directed towards some 

 common point so as to form a cone. Here then was cone-in-cone 

 structure produced by the arrangement of the crystal fibres of calcite. 



Carbonate of lime, carbonate of iron, and carbon itself in the 

 form of coal, exhibit this structure, often on a much larger scale 



1 It shovild be noted that these marvellous surface contortions and other markings, 

 so characteristic of the Mid Wales rocks, are convex on the unde?- surface of the 

 flags and grits. 2 Mostly seaweeds. 



