concerning the Structure of the Earth. 235 
39. Beside the horizontal division of the earth into strata, 
these strata are again divided and shattered by many perpendi- 
cular 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 almost no 
fissures at all, though the strata both above and below it are 
considerably 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 superincumbent matter, and fill up those 
fissures which it originally had(c); for we sometimes meet with 
imstances 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. 
40. 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, how- 
ever, in which the larger veins of the ores of metals are found, 
are an exception to this observation; for they sometimes pass 
through many strata(d), and those of different kinds, to unknown 
depths. | 
4], From 
whether he had noticed, and meant to include herein, the thin lamine, 
highly inclined to the real strata, and called by me stratu/a, which some- 
times occur in the Sandstones of the Coal-measures and are very frequent 
in the coarse Slate districts. (Derby. Report, 1. 155.)—J. F. 
(c) In this 39th section, and the beginning of the 40th, it is to be ob- 
served, that Mr. M. is speaking exclusiy ely ‘of joints or cutters, dividing 
the individual thin strata or beds, into separate blocks, and not of Mineral 
Veins (§ 40), or of Faults (§ 50); although, like as with the latter, he sup- 
poses, that these joints passed at one period through some of the contiguous 
strata or beds, but have since disappeared from the softer ones, by the 
pressure of the incumbent beds, having again closed up the joints in them : 
this latter, however, appears an unnecessary and unwarrantable supposition: 
the face of hardly any deep Quarries or of Cliffs can be contemplated, with- 
out perceiving, that scarcely any two, even of the adjacent beds of Stone, 
have joints correspending with each other, either in the width of open 
fissure, or in their places; evidently, because the individual beds, have, by 
their own peculiar degree of contraction or shrinking, opened these joints, 
in various and independent degrees; aud very frequently, Strata or Beds 
may be found, between others intersected by joints, which have themselves 
suffered no lateral contraction, and are, and ev idently always have been, 
without any joints: thisis very common with strata of Clay between others 
of Limestone, and between Sandstones, with Basaltic Strata interlaying 
Limestones, like those of the Peak-hundreds of Derbyshire, &e.J. F. 
(d) Although Mineral Veins, including Dykes or those solidly filled with 
Basalt or other stony matters, pass in most cases “ through many strata,’ 
their fissures will, ona careful examination appear to have been, not less cer~ 
tainly opened by mere shrinking, thau the joints spoken of in my last Note: 
the Vein Fissures, appear to have preceded the joints, in the order of open- 
ing, and to have resulted from causes or tendencies to shrink, more’ ex- 
tensive and symmetrical, as is evinced by the length, parallelism and width 
of 
