426 CRITICAL NOTICES OF NEW PUBLICATIONS. 



been cracked interiorly by induration. The ironstone nodules, although 

 tolerably round, shew no marks of attrition : all over them are indenta- 

 tions resembling the eyes of potatoes : they lie in horizontal rows, 

 regularly arranged, in the stratum which contains them ; and they, as 

 well as the rounded portions, have concentric layers, with a nucleus of 

 some organic substance. 



In the Ashby coal-field, there are numerous seams of fire-clay : this is 

 very pure, and contains many remains of aquatic plants. When 

 subjected to a heat of 6500^ F. or 40« of Wedgwood's thermometer, its 

 colour announces a slight impregnation of iron. More than ten thousand 

 dozens of yellow pottery ware, manufactured from this fire-clay, are 

 sent weekly from Ashby Wolds to all parts of Great Britain, America, 

 and the West Indies. Having completed his descriptive statements, 

 Mr. M. proceeds to offer some instructive remarks, and these are greatly 

 distinguished by his characteristic sagacity. " From the circumstance,'* 

 he says, p. 73, " that so many instances occur where pure fire-clay lies 

 immediately under, and in contact with, a bed of coal, we may infer 

 that this clay could not have been the soil where grew the vegetable 

 matter which produced the coal, unless this vegetable matter was a 

 moss, a peat, or some aquatic plant ; because, in the clay, there is no 

 appearance of roots, or trunks, or other vegetable impressions beyond 

 slender leaves, as of a long grass. The fact, that particular strata 

 accompany the main coal for many square miles, would support the idea 

 that an immense flat was originally covered with the substance of this 

 fire-clay many feet thick ; and that, upon this flat, there took place an 

 uniform growth of a distinct single vegetation which must have occupied 

 the position for a long period, and thus furnished the matter whereof 

 the main coal is composed. The alternations of fire-clay and coal-seams 

 would favour the notion, that their materials were originally mixed 

 together in a fluid ; and that those of the former, by their gravity, 

 would first subside ; whilst the vegetable matter, or those of the former, 

 would undergo a more gradual and quiet deposition : hence, by a 

 repetition of the process, the alternations of the strata would be pro- 

 duced. Besides, if the strata of coal had derived their origin from the 

 growth and destruction of a forest, some portions of them would have 

 been thicker than others, or altered in quality, or have retained at least 

 some trace of forest trees ; whereas, on the contrary, the most extra- 

 ordinary uniformity in quality, compactness, and thickness of the 

 seams, prevails to a great extent.'* 



Chap. XL — For the purpose of fixing a mean temperature of the 

 Ashby mines, Mr. M. instituted a series of experiments, and these with 

 their results are here exhibited in a tabular form. They bear manifest 

 evidences of having been conducted with unusual exactness. Intro- 

 ductively, he notices the supposition that the earth acquires an increase 

 of heat, in some ratio of descent from the surface ; and, at the same 

 time, he admits that the difficulty of establishing this supposition is 

 almost insurmountable. Deductively on this branch of his subject, he 

 observes, p. 76, ** upon consideration of the various circumstance under 

 which the temperature of a mine is taken, whether in the solid earth, in 

 crevices closed up, in a stream of issuing water, or on the atmosphere of 

 the mine itself, there are no data upon which any increase of heat can 

 be assured to be a consequence of increase of depth. From the fact 

 that 448 to 46° F. indicates the ordinary heat of water in these mines, it 

 may be inferred that this is the true temperature of the earth, at 

 similar depths. It is never found lower than 44« F., and in all the 

 variations above this, it is not diflficult to trace their cause." ** The water. 



