Murchisoris Silurian System. 547 



and other popular works ; and as to the other three species of 

 the same genus they depend upon fragments which would 

 hardly be intelligible to any one but M. Agassiz himself. 

 They appear however to indicate a genus of considerable 

 variety and extent, all the remains of which are confined to 

 these beds, but not to this locality; for Dr. Malcolmson, 

 who distinguished himself by his geological and medical 

 researches in India, also discovered the remains of Cepha- 

 lopsous fishes in Scotland, in beds of calcareous conglome- 

 rate, which he regards as equivalent to the cornstone of 

 England. We now come to the third, or lowest member of 

 the old red system, named by Mr. Murchison 



Tilestone. 



This lowest member of the group has very distinct cha- 

 racters both in structure and fossil contents from either of 

 the foregoing divisions of the old red sandstone. Its upper 

 beds pass into cornstone and marls, such as we have first 

 described ; while the lower beds change into Silurian rocks. 

 The tilestone runs in nearly a straight course from a place 

 called the Bridge of Tiles in Carmarthenshire to near Builth, 

 on the north east, occupying the loftiest part of the escarp- 

 ments of wild mountainous tracts, at heights of fifteen to 

 sixteen hundred feet. In this range the tilestones are ex- 

 tensively quarried, and the strata which inclined at angles 

 from 45° to 80° dip invariably to the south-east. After a 

 great flexure on the Wye to east of Builth, the tilestones are 

 again found in similar relations, overlapping the Silurian 

 rocks in the Clyro and other hills in Radnorshire, and ex- 

 tending from thence to Kington in Herefordshire, in which 

 part of their range they are much less inclined. Through- 

 out their course from Caermarthenshire to Kington the dis- 

 tinguishing beds are finely laminated, hard, reddish or green 

 micaceous quartzose sandstones, which split into tiles. Al- 

 though the greenish colours prevail, these beds are usually 



4 B 



