IOO 



HARDWICKE'S SCIENCE-GOSSIP. 



from Cardiff. Caerphilly, with the ruins of its 

 ancient castle, lies to the north of, and is distant 

 from Cardiff about seven miles. A walk over the 

 Rhymney Railway reveals the following section : 

 After passing over the alluvial deposits at Cardiff, we 

 enter a cutting through a bed of river gravels ; this 



8 feet. 



10 feet. 



9 feet. 



1 8 feet. 



24 feet. 



30 feet. 



Ammonites planorbis beds. 



Lima beds. 



Ostrea liassica beds. 



I' HI i' 1 ,'l", 



I I Mi 



~r-- - 1 



■1 N , * * .rf a«_/ White Lias series. 



»fc- ;..Sa»3l 



Black Shales. 



> j 1 1 1 1 |i 



Green Marls. 



Red 

 Marl. 



Fig. 89.— Section of Rhsetic beds at Penarth. 



is immediately followed by a cutting in the Silurian, 

 and is the most westwardly exposure hitherto dis- 

 covered in this district ; it is, in fact, a prolongation 

 of the base of Pen-y-lan referred to above. A heavy 

 embankment occurs for about a mile, which brings 

 us near Llanishen, where we enter a fine section of 

 the Old Red Sandstone, consisting of conglomerate 

 pebble beds, beds of grey and red sandstone alternat- 

 ing with beds of similar coloured marls ; these are 

 succeeded by the carboniferous limestone, millstone 

 grit, and lower coal measures. 



These rocks at once mark a significant change in 

 the features of the landscape ; rising to over 850 

 feet above sea level, they form the Caerphilly 

 mountain, under which the Rhymney Railway is 

 carried by a tunnel 1760 yards in length. In walking 

 over the mountain we pass over the denuded and up- 

 turned edges of the Old Red Sandstone, carboniferous 

 limestone, millstone grit, and, on the northern slope, 

 between the summit and the town of Caerphilly, 

 twelve seams of coal, alternating with beds of sand- 

 stone and shale, crop out. About two miles further 

 to the north, the great anticlinal axis which divides 

 the South Wales coal basin into two unequal troughs 

 is met with. It is composed of Pennant sandstones ; 

 these furnish good building material, and from some 

 of the beds an excellent paving-stone is obtained. 

 The thickness of these sandstones, as ascertained 

 from sinkings, is over 480 feet. 



The currents which brought the material for the 

 formation of these rocks also brought large portions of 

 the vegetation of the period, which have been well 

 preserved in a quarry at Pwll-y-pant ; some of the 

 beds are literally crowded with such remains. When 

 first exposed the external portion of the wood is 

 found converted into pure coal having a cubical 

 fracture inside, the wood has been fossilized, and an 

 excellent idea of its structure may be obtained by 

 preparing thin sections for microscopical examination. 

 By far the larger portion of these remains consist of 

 Sigillaria, but from the fact of Dadoxylon antiquus, 

 Pinites, Lepidodendron and Psaronius having been dis- 

 covered, a careful examination would doubtless reveal 

 to an investigator of fossil botany many other descrip- 

 tions of the flora of the coal age. 



That these remains have been subjected to consider- 

 able attrition is indicated by the ends of each piece of 

 wood terminating in a blunt point, while not un- 

 frequently patches of fossilised vegetable matter may 

 be found. These are undoubtedly the comminuted 

 fragments worn from the stems and branches during 

 the turbulent state of the water, and held in suspen- 

 sion until it became more tranquil, when they were 

 finally deposited in the slight inequalities of the sea 

 bottom. 



The general arrangement of the beds in this section 

 is briefly as follows : when the Silurian is first met with 

 the dip is about 23 degrees to the N.E. The embank- 

 ment referred to occupies a depression where the beds 



