C— GEOLOGY 65 



<'i per cent. ; but this does not merely imply the loss of 75 or 80 per 

 cent, of the other elements, for the oxygen and hydrogen have gone off 

 largely in combination with carbon. What the gross amount may be 

 I do not venture to say, but my opinion is that the reduction in passing 

 from the state of wet undisturbed peat will not be much less llinn 15 or 

 20 to 1.' 



Let us now, with these facts in mind, return to the consideration 

 of the plano-convex lens of ' dirt ' occupying a position between the 

 upper and lower elements of the split seam at Whitwood. On the sag 

 explanation it should be convex downward, yet in -this as in all other 

 cases I have investigated, it is convex upward. The explanation is 

 simple. Let us make our mental picture of the infilled channel in the 

 peat a little more specific in detail. Let us suppose that the peat was 

 40 feet in thickness when the river commenced to cut its course across it ; 

 the channel we will say was, like most channels, deeper in the middle 

 than at the sides, and in the middle actually cut through to the seat- 

 earth. Then the channel silted up completely, so that a cast of its 

 meandering course in sands or mud reaching 40 feet in thickness at the 

 maximum, but much thinner at the margins, was formed, then the upper 

 bed of peat formed to a fm-t.her depth of 40 feet. The conversion of the 

 peat into coal would reduce it to two beds, each, let us say, 2 feet in 

 thickness at the, maximum, enclosing the sediment with a proportionately 

 smaller thickness in the eroded peat on either margin of the channel. 

 The sedimentary mass would have the transverse section of a plano- 

 convex lens, the convexity being downward, but when the peat under 

 the edges of the sediment is condensed to one-twentieth of its original 

 bulk the base becomes almost flat, and the unconsolidated mass of sedi- 

 ments adjusts itself thereto. Thus the curve, originally at the base of the 

 mass, reproduces itself in the top of the mass, which was originally 

 quite flat and now is curved. The lens of infilling has reversed its 

 curvature. 



In the Castle Comer Coalfield, County Kilkenny, I have been able 

 to examine underground an almost exactly similar case of a portion 

 of a horseshoe-shaped meander exhibiting the same reversal of the lens, 



6 I take this opportunity to expose a fallacy of very wide acceptance. It 

 appears to be a general belief that, as in Coal Measure rocks pebbles of coal 

 occur which are closely embraced by the matrix, and similarly that the shell of 

 coal surrounding a standing tree-trunk is in contact with the matrix both within 

 and without, therefore no appreciable reduction of bulk of the vegetable interior 

 took place in the process of ' coalification.' The assumption here made is that 

 the surrounding rock attained complete induration prior to the accomplishment 

 of that change in the enclosed masses of vegetable matter, yet all analogy 

 forbids that supposition. The Mid-Encene beds of Alum Bay and Bourne- 

 mouth, though quite incoherent, contain thin coals as bright and lustrous, as 

 truly ' coalified.' as many of our Carboniferous coals, yet it would hardly be 

 contended that the period that has elapsed since their formation is materially 

 less than the duration of Coal Measure times. The evidence points to the proba- 

 bility that the accomplishment of the greater part of the change from plant to 

 coal took place while the measures were still unconsolidated, and were able 

 to adjust themselves to the shrinkage of the contained masses of coal-stuff. 

 When I come to speak of the cleavage of coal a further argument will emerge 

 in favour of the consolidation of the ' measures ' being subsequent to that of 

 the coal. 



G 2 



