Murchison 's Silurian System. 229 



says that millstone grit consists of pebbly, quartzose con- 

 glomerate and thick-bedded hard sandstone, which rises 

 at many points from beneath the productive coal field. 

 It is most expanded in the sterile tract which lies between 

 the northern slopes of the basalts and the limestone hills at 

 Oreton. When the limestone is wanting, the millstone 

 grit rests immediately on the old red sandstone. (See p. 118.) 

 In Oswestry coal field the millstone grit or peculiar sand- 

 stone of the coal measures is largely developed, rising into 

 broad ledges between the productive coal field and higher 

 hills of limestone, from whence it turns round the coal field, 

 cutting off the productive districts from the older rocks. 

 In most situations the strata dip at very slight angles be- 

 neath the coal, rarely however they are much inclined. 

 From Oswestry the millstone grit advances in low hills to 

 the edge of the great plain of Shropshire, and exhibits 

 the following succession of strata in the descending order : — 



1. Light coloured siliceous sandstone, containing a stratum some 

 feet thick, of a porous rock made up of fragments of chert, imbedded in 

 a matrix of fine white clay, or decomposed felspar and silex (kaslin). 

 This bed resembles that which occurs on a larger scale in the north- 

 western prolongation of these carboniferous tracts at Halkin in Flint- 

 shire. It is here underlaid by whitish or pinkish sandstones, sometimes 

 freckled with spots of decomposing oxide of iron : other and lower beds 

 forming the summit of Sweeny mountain are coarser, containing dis- 

 tinct pebbles of quartz. The finer varieties of these siliceous sandstones, 

 whether of whitish pink or deep red colours, afford excellent building 

 stones, and are capable of being wrought into the ornamental parts 

 of architecture. 



This millstone grit, with its light coloured and whitish building- 

 stone, ranges over the grounds of Porkington, rising up in large masses 

 to Sallattyn mountain, where it rests upon the limestone. Towards the 

 bottom of the formation these sandstones become partially calcareous, 

 and present a honey-combed aspect, due to the unequal disintegration 

 of their surface. Occasionally the rock may even be termed a sandy 

 limestone. Fragments of encrinites and corals are also found in these 

 beds, announcing their approach to the calcareous masses beneath. 

 Such masses of calcareous red sandstone are seen also at Ponty-Cefn, 

 occupying a broad zone between the limestone and the productive coal- 



