144 MILLSTONE GRIT AND SANDSTONE OF SWEENY AND SALLATTYN. 



round the productive coal-field, and reducing it to the small embayed area indicated in 

 the Map. In most of these situations the strata of which they are composed, dip at 

 very slight angles beneath the coal, but in the hills of Mynidd Myfwr, and on Oswestry 

 racecourse, the beds are highly inclined. From Sweeny Mountain the same strata 

 advance in low hills to the edge of the great plain of Shropshire, and exhibit the fol- 

 lowing succession in descending order. 



1st. 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 (kaolin). This bed resembles that which occurs on a larger scale in the north-western 

 prolongation of these carboniferous tracts at Halkin in Flintshire 1 . 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 distinct 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. (See the recent restorations of Oswestry church.) 



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 be- 

 come partially calcareous and present a honeycombed aspect, due to the unequal disin- 

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

 stone (d. of section). Fragments of encrinites and corals are also found in these beds, 

 announcing their approach to the calcareous masses beneath. Such masses of calca- 

 reous red sandstone are seen also at Pont y Cefn, occupying a broad zone between the 

 limestone and the productive coal-field. They are, however, completely separated 

 from the underlying limestone by a very thick development of pure sandstone, often 

 of a deep red colour, and they may therefore be considered as subordinate to the mill- 

 stone grit. The red sandstone, or lowest member of the formation, is well exposed 

 to the east of Sweeny Mountain, resting directly upon the great carboniferous lime- 

 stone, in which position the red rock has the thick-bedded structure, and exact appear- 

 ance of many varieties of the New Red Sandstone, thus affording one of the numberless 

 examples which will be found in this work of the impracticability of defining the age of 

 strata by mere lithological aspect (e. of section) 2 . 



1 In Mr. A. Aikin's manuscript notes, I perceive that he describes this bed in other parts as being composed 

 of knots and reniform concretions of chert, and the white matter as fine granular silex. The Rev. J. Yates has 

 also given a good lithological description of these grits, though he is in error respecting the geological position 

 or age of these rocks, which he supposed to form a part of the New Red Sandstone of Shropshire. (Geol. Trans., 

 vol. ii. New Series, p. 240.) He describes some of these upper beds as containing hornstone, passing into per- 

 fect flint, and he further gives an account of the succession of strata downwards to the limestone, showing at 

 Mynidd Moel how the passage from the one group into the other is effected through a bed composed of grains 

 and small pebbles of quartz and slate, joints of encrinites, &c. 



9 I am informed by Mr. Bowman of Gresford, near Wrexham, that the millstone grit of the Hope Mountain 



