454 PHILOSOPHICAL TRANSACTIONS. [aNNO I76O. 



their neighbourhood, or that there are at present subterraneous fires near 

 them. 



This frequency of subterraneous fires, in the neighbourhood of volcanos, will 

 appear still more probable, if we consider the internal structure of the earth; 

 and as it will be necessary also, in order to understand what follows, to know a 

 little more of this matter than what falls under common observation, he gives the 

 reader some account of it. 



The earth (as far as one can judge from the appearances) is not composed of 

 heaps of matter casually thrown together, but of regular and uniform strata. 

 These strata, though they frequently do not exceed a few feet, or perhaps a tew 

 inches in thickness, yet often extend in length and breadth for many miles, and 

 this without varying their thickness considerably. The same stratum also pre- 

 serves a uniform character throughout, though the strata immediately next to 

 each other are very often totally different. Thus, for instance, we shall have 

 perhaps a stratum of potters* clay; above that a stratum of coal; then another 

 stratum of some other kind of clay; next a sharp grit sand stone; then clay 

 again; next perhaps sand stone again; and coal again above that; and it fre- 

 quently happens that none of these exceed a few yards in thickness. There are 

 however many instances in which the same kind of matter is extended to the 

 depth of some hundreds of yards; but in all these, a very few only excepted, the 

 whole of each is not one continued mass, but is again subdivided into a great 

 number of thin laminae, that seldom are more than 1, 2, or 3 feet thick, and 

 frequently not so much. Beside the horizontal division of the earth into strata, 

 these strata are again divided and shattered by many perpendicular fissures, which 

 are in some places few and narrow, but oftentimes many, and of considerable 

 width. There are also many instances where a particular stratum shall have al- 

 most no fissures at all, though the strata both above and below it are consi- 

 derably broken: this happens frequently in clay, probably on account of the 

 softness of it, which may have made it yield to the pressure of the superincum- 

 bent matter, and fill up those fissures which it originally had ; for we sometimes 

 meet with instances in mines, where the correspondent fissures in an upper and 

 lower stratum are interrupted in an intermediate stratum composed of clay, or 

 some such soft matter. Though these fissures do sometimes correspond to one 

 another in the upper and lower strata, yet this is not generally the case, at least 

 not to any great distance: those clefts however in which the larger veins of the 

 ores of metals are found are an exception to this observation ; for they some- 

 times pass through many strata, and those of different kinds, to unknown depths. 



From this constitution of the e^irth, viz. the want of correspondence in the 

 fissures of the upper and lower strata, as well as on account of those strata which 

 are little or not at all shattered, it will come to pass that the earth cannot easily 



