South-western Coal District of England. 221 



the frontier of this basin, since it only occurs near Tortworth without its 

 northern apex, and is there immediately connected with extensive masses of 

 the same rock, which form part of the northern frontier of the basin of the 

 Forest of Dean. To the chapter therefore on the latter basin, and to the 

 memoir by Mr. Weaver on the Geology of the Neighbourhood of Tortworth, 

 we refer for particulars concerning this rock. 



3. Old red sandstone. 



This formation consists of various alternating beds of sandstone, quartzose 

 conglomerate, and slaty marl usually much indurated. These are generally 

 of a brick-red colour, often flecked with streaks or mottled with spots of a 

 yellowish green colour. Sometimes entire beds are of a yellowish green, or 

 greenish grey hue. 



The sandstone usually is composed of fine grains of quartzose sand, mixed 

 in variable proportion with red clay and mica, white specks of felspar being 

 frequently disseminated through it. The mica, when very abundant, is dis- 

 posed in rapid alternation with thin laminae of argillaceous sandstone, then 

 producing that fissile character which is prevalent in the beds of this forma- 

 tion, and occasions them to afford flagstone and coarse tiles. Except in the 

 case of the most fissile beds, the sandstone is extremely hard and tough. 



The conglomerate contains pebbles of milky quartz ; occasionally of red and 

 black jasper ; and more rarely of slate. These are imbedded in a matrix of 

 sharp, glassy, quartzoze sand, firmly cemented by red oxide of iron. 



The sandy strata alternate frequently with beds of dark-brown tenacious 

 clay, which in Herefordshire and in the vale of Monmouthshire assumes the 

 form of a brick-red marly clay, and there gives a fertility to the old red sand- 

 stone formation as strongly characterized as is its barrenness in the Mendip 

 chain. This sterile character is, indeed, exaggerated in the Mendips by their 

 high elevation and exposure ; the change of stratum, however, from limestone 

 to sandstone is immediately indicated throughout the chain by a change of 

 herbage ; the surface acquiring a swampy appearance, even on the steepest 

 slopes, wherever the sandstone begins to prevail. Occasionally a few fertile 

 spots may be found upon the sandstone ; but this is generally owing to the 

 presence in the subsoil, whether it belong to the old or the newer red sand- 

 stone, of that peculiar red marl which forms the basis of the richest soils in 

 the most luxuriant parts of England. 



The old-red-sandstone is known throughout the district by the name of* 

 firestone, from its being employed for making hearths. In the old red 

 sandstone of this basin only one occurrence has been noticed of those cal- 

 careous concretions which are so prevalent in the sandstone of Hereford- 



