284 Messrs. Buckland's and Conybeaue's Observations on the 



disposed in balls and knobs of a lead-blue colour, imbedded in sbale. Where 

 the limestone alternates with sandy beds, it passes gradually into a calcareous 

 sandstone. 



3. Old red sandstone. 



The old red sandstone, whose limits are so restricted in other parts of 

 England, here occupies an extensive area, being a continuation of that greater 

 tract, over which, in Brecknockshire and Herefordshire, the same formation 

 is spread. The space which it here covers, its great thickness, its high in- 

 clination, the abrupt character of the surface over which it prevails, and the 

 consequent display of its strata in many natural sections, present in this di- 

 strict advantages for studying the formation, which are not to be met with 

 elsewhere in South-Britain. In the neighbourhood of Mitchel Dean, ac- 

 cording to the section of Mr. Mushet, which we ourselves have verified, the 

 total thickness of this formation, interposed conformably between the transi- 

 tion and mountain limestone, is from 600 to 800 fathoms. 



It appears practicable to subdivide the formation into three distinct portions. 

 In the lower portion, compact micaceous slaty sandstone predominates, ap- 

 proaching in character to the coarser varieties of grey-wacke slate, and oc- 

 casionally becoming a red calcareous slate. The abundance of highly- 

 indurated ferruginous clay, and of mica, in the sandy beds of this formation, 

 renders them harder and more fissile than those of the newer red sandstone; 

 and thereby qualifies them for flag- and paving-stone ; but at the same time 

 renders them a very indifferent free-stone. These lower beds, as they ap- 

 proach the transition limestone, contain the organic remains of that forma- 

 tion. The middle portion of the old red sandstone consists of marl or marly 

 sandstone, through which are disseminated irregular, concretional, pebble- 

 shaped, nodules of limestone, giving to the rock the appearance of a breccia. 

 These masses occur in irregular proportions, and vary in size from that of 

 a pea, to blocks of many tons. This pseudo-conglomerate is generally 

 known in Herefordshire by the name of cornstone. For a minute account of 

 the Herefordshire cornstone, and for the characters which distinguish it from 

 a true breccia, we refer to a note in Professor Buckland's paper on the 

 Lickey Hill, page 512, vol. 5, old series, Geol. Trans. 



The cornstone may be seen at the western base of the Kyming-hill, to the 

 east of Monmouth, and occurs extensively between Monmouth and Aber- 

 gavenny, and in the hills of old red sandstone, to the east of Newport, on 

 the road to Christchurch, in Monmouthshire. It is not unfrequently worked 

 for lime ; but from the prevalence of limestone of better quality in the neigh- 

 bourhood of the Forest of Dean, it is there an object of less importance than 



