540 Murchiso?i's Silurian System. 



would ill supply the place of that work itself, but we trust it 

 will prove useful to such as cannot have recourse to tne ori- 

 ginal in India ; we shall therefore endeavour to give the 

 reader as practical and concise a view of Mr. Murchison's 

 details as possible. The second division of the old red sand- 

 stone is the 



Central, or Cornstone Formation. 

 The central masses are chiefly composed of alternations 

 of red and green argillaceous spotted marls, affording rich 

 soil. These argillaceous beds sometimes alternate with sand- 

 stone, but more frequently with irregular courses of concre- 

 tionary impure limestone, mottled also red and green. When 

 compact the calcareous beds are termed limestones by the 

 quarry men, but when mixed with sand and marl, giving 

 them a brecciated or conglomerate aspect, they constitute 

 the well known cornstone of Herefordshire and the adjoining 

 counties. 



" We may commend the detailed description of this formation in 

 Carmarthenshire, because, although the cornstones in that county are 

 of very impure quality, their relations are clearly exposed in a section 

 exhibited in descending from the Carmarthenshire fens towards the 

 bridge called Pont-ar-lleche. The river Sowdde here runs in a nar- 

 row cleft or channel, which cuts the strata at right angles to their 

 strike. The inclination of the beds is towards the south-east, and the 

 dip varies from 65° near the superior limits of the formation, to 75° 

 towards its base, or junction with the tilestones. In the short distance, 

 therefore, of about three-quarters of a mile, we obtain by means of 

 the high inclination of the strata, and the clearness of the section, 

 a perfect knowledge of all the beds comprising the cornstone division, 

 which, owing to their slight inclination and gentle undulations, are 

 expanded over the low and fertile tracts of Brecknockshire and Hereford- 

 shire, and are there rarely well exhibited as a whole. The strata consist 

 of deep red shale, argillaceous sandstone, and hard, quartzose, dingy 

 purple, or brown sandstone, slightly micaceous ; with interculated 

 calcareous beds, of a concretionary and pseudo-concretionary structure. 

 These calcareous concretions, varying in colour from red to green, and 

 in diameter from half an inch to three or four inches, are disseminated 



