144 



WALTER KEEPING ON THE 



Mountains (figs. 1 & 2). The lowest beds form the cliffs at the coast, 

 while the highest go to form the central mountains. All these rocks 

 are amazingly contorted ; and we find the intermediate or Metallifer- 

 ous group especially thrown into frequent and violent twistings, 

 with, in many places, actual inversion. A continuation of the section 

 eastward, beyond Plynlimmon (fig. 2), shows a similar appearance of 

 the rock groups in reversed order of outcrop and with contrary dips, 

 thus showing that Plynlimmon, like its more ancient and greater 

 fellow Snowdon, stands in a geological valley or synclinal. We 

 proceed to the detail of this section. 



1. The Aberystwyth grits consist of beds of hard, compact, dark 

 grey grit or greywacke, and dark shales, rabs, and imperfect slates 

 in strikingly regular alternation, as may be seen in the ordinary 

 photographs of the Aberystwyth cliffs. 



The grey grits or greywackes are of great sameness and regularity 

 both in structure and composition around Aberystwyth, being hard, 

 grained rocks, often felspathic, regularly and sharply jointed. A 

 cross fracture often shows a remarkable contortion in the lines 

 of laminae, this being, I believe, mostly of subsequent " concre- 

 tionary " origin ; many of the beds themselves are also, in part, 

 of the same concretionary growth. The beds measure very con- 

 stantly about 4 to 6 inches in thickness ; and their under sur- 

 faces exhibit an abundance of raised markings, which are irregular 

 or tortuous, branching, net-like or worm-like, these being also, in 

 part, of concretionary origin. 



The argillaceous partings are usually of about the same thickness 

 as the greywacke-beds, sometimes thicker (especially to the east and 

 south), sometimes thinner (as in many places around Aberystwyth). 

 Most of their varieties are the result of subsequent metamorphic 

 changes acting differently upon the rock according to a slight 

 diversity of original constitution, or depending upon slightly dif- 

 ferent mechanical conditions. Thus have been produced the various 

 forms of shivery shale, large platy shale, rubbly rab* of various 

 forms and sizes, soft shaly slate, and even very well-marked regular 

 slate in the Aberystwyth district. They are uniformly of dark 

 colour, and never greatly indurated. Lenticular nodules with " cone- 

 in-cone " structure are of frequent occurrence both in this and the 

 following (or Metalliferous) series. 



Fossils. — Fucoidal and worm-like markings are of frequent and 

 wide-spread occurrence throughout this series, appearing for the 

 most part in the form of raised markings upon the under surfaces of 

 the grits. I have also found Graptolites in several localities. 



Quarry at Cwm, on the south side of the Clarach Valley. 



Monograptus Sedgewickii, Portl. 



Clingani, Carr. 



lobiferus, M'Coy. 



Monograptus tenuis, Portl. ? 

 Buthotrepkis, small species. 



* A rah is a fine-grained rock, usually argillaceous and not indurated, which 

 readily breaks up into a rubble of cuboidal or prisrn-like fragments. 



