230 Murchison s Silurian System. 



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 millstone grit. The red sandstone, or lowest member of the forma- 

 tion, is well exposed to the east of Sweeny mountain, resting directly 

 upon the great carboniferous limestone, in which position the red rock 

 has the thick bedded structure, and exact appearance of many varieties 

 of the new red sandstone, thus affording one of numberless examples 

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

 the age of strata by mere lithological aspect. 



As the red variety of the millstone grit to the east of Sweeny is 

 in juxtaposition with the lower new red sandstone, the exact line of 

 demarcation between the two formations becomes difficult, particularly 

 where the surface of the lower grounds is much encumbered by detritus. 

 The apparent similarity of the red rock of the millstone grit to that of 

 the lower new red being likely to mislead speculators who do not 

 comprehend the structure of the district, I would therefore repeat, that 

 the first mentioned rock distinctly underlies the coal measures, and reposes 

 on the limestone, and hence that any effort to seek for coal beneath it 

 would be absurd. 



Nothing could add to the importance of this last observa- 

 tion, or point out more clearly than the foregoing remarks 

 of Mr. Murchison, the advantage of a scientific knowledge 

 of rocks in the examination of districts in search of coal, 

 as well as in all operations that may be had recourse to 

 for its recovery. 



The following observations on the character of coal, 

 and the various forms the mineral assumes in the same 

 district, is of much practical importance, and should point 

 out the error of deciding upon the quality of coal from 

 specimens alone, unless we know them to be taken from 

 the main bed. Where there are numerous mines in a dis- 

 trict the main coal will be well known, but in a new coun- 

 try where no information is to be had, but what is to be 

 derived from natural sections, the difficulty is only to be 

 overcome by geological investigation, and however sceptical 

 we may be as to the practical value of geological science, 

 the authority of Mr. Murchison ought to be a sufficient 



