OLD RED SANDSTONE. 11/ 



' cornstones ' occur ; and ripple-marks are met with on the 

 surfaces of some of the beds of sandstone. The maximum 

 thickness of the series may be taken at about io,oco feet, but 

 in many places it would appear that an estimate of from 3000 

 to 5000 feet is sufficient. 



The Old Red Sandstone extends from near Bridgenorth in 

 Shropshire, southwards, through a considerable portion of Here- 

 fordshire, Monmouthshire, and Brecknockshire, into Glamorgan- 

 shire, Caermarthen, and Pembroke. 



Over the greater part of this area the Geological Survey Map was constructed 

 by Sir Henry T. De la Beche, W. T. Aveline, and Trevor E. James. 



In the Forest of Dean the thickness of the Old Red Sandstone has been 

 estimated at 8coo feet ; and Mr. H. Maclauchlan, who surveyed much of the 

 country geologically, remarked on the difficulty of separating the Old from the 

 New Red Sandstone.^ 



Three divisions of the Old Red Sandstone have sometimes been 

 adopted, as follows: i. Cornstone or IMarl Series ; 2. Flagstone 

 Series ; and 3. Conglomerate Series." 



For a threefold division of the Old Red Sandstone Murchison was 

 responsible (although he at first included the ' Tilestone Group'), 

 for he had so divided the beds in Scotland, where, since the date 

 of the writings of Hugh INIiller, a charm has become attached to the 

 name Old Red Sandstone. Dr. A. Geikie, however, has stated 

 that he was never able to resist the suspicion that, plausible 

 though the argument from fossil evidence appears, the threefold 

 subdivision was unconsciously suggested by the seemingly well- 

 established threefold arrangement of the true Devonian rocks, 

 and by the natural desire to establish a closer analogy between 

 these rocks and the Old Red Sandstone. His own work in the 

 centre and south of Scotland proves the Old Red Sandstone to 

 consist of two great divisions, — a lower passing down conformably 

 into the Silurian rocks, and an upper graduating into the Carbon- 

 iferous rocks, with a complete discordance between the two 

 divisions.^ 



In Ireland, too, a similar division of the Old Red Sandstone has 

 been made, of which the lower portion comprises the well-known 

 Dingle Beds or Glengariff Grits. JMoreover, Jukes in 1857 had 

 become convinced that the Old Red Sandstone of South Wales 

 was made up of two series, although he thought they were con- 

 formable near Abergavenny, but towards Llandeilo there was either 

 unconformity or a number of faults along the strike.* Indeed, 

 the marked overlap of the higher beds of the Old Red Sand- 

 stone (which pass up into the Carboniferous series), so well shown 

 on our geological maps, suggests a discordancy in the middle of 



> Proc. G. S. i. 421. 



- Phillips, Manual of Geology, 1855, p. 141 ; Symonds, Records of the Rocks, 



p. 212. 



^ Trans. R. Soc. Edin. xxviii. 347. 



* Letters, etc., 1871, p. 508. See also G. H. Kinahan, Geology of Ireland, 

 p. 50. 



