64 SECTIONAL ADDRESSES. 



interruption to be mapped continuously for about 8 or 9 miles and 

 intermittently much further. 



My first impression was that this was just a simple case of Bowman's 

 'sag,' until I observed that in every traverse the vpper element of the 

 seam was arched while the floor was flat. 



Several analogous cases came under my notice before an explanation 

 of this anomalous arching was reached. The explanation was found to 

 lie essentially in the differential shrinkage undergone by peat-stuff in 

 the process of forming coal, and, on the other hand, by any sand or 

 mud which may have been deposited so as to replace a part of the peat. 



Let us imagine a stream being diverted at flood time across a bed 

 of peat and scooping out for itself a hollow channel, which channel 

 subsequently becomes filled with sediments, after which the formation 

 of peat continues, the peat plants creep out, and presently envelop the 

 whole mass of sediments. When the beds consolidate there will 

 obviously be very different contraction between the sands, muds, and 

 the. cool-stuff. The sands, as I have said, will hardly contract at all, the 

 muds will contract a good deal, the coal-stuff will contract very greatly. 



Various estimates — or guesses — have been made of the amount of 

 reduction in bulk which attends the conversion of peat into coal. 



Lomax shows that where coal-balls — which are really masses of com- 

 paratively uncrushed coal-forming material that has been preserved by 

 minerals infiltering the tissues and the interspaces — occurred abundantly, 

 the seam, including the coal, became thicker according to the quantity 

 O'f coal-balls present. Where a large number were massed together the 

 seam became more than 6 feet thick, while on every side the coal was 

 not more than a foot thick. Again, he says ' a large mass of petrifac- 

 tions was found, and which, although more or less crushed by superin- 

 cumbent weight, retained a he'ight of 7 feet 3 inches, while the corre- 

 sponding layer of coal was only 10 inches thick.' He estimated the 

 loss by flattening out at one-third ' so that it might be estimated that 

 11 or 12 feet of vegetable matter had been deposited to form one foot 

 of coal.' ' 



I have found that dry peat can be compressed in a testing machine 

 to one-fifth of its original thickness, and making allowance for the loss 

 in drying, and for the great reduction of bulk attendant upon the change 

 from peat to coal, I am disposed to set a still higher value than Lomax 

 on the reduction. It should be borne in mind that wood has an average 

 of about 50 per cent, of carbon and 50 per cent, of hydrogen, oxygen and 

 nitrogen, while the carbon in an average: house-coal ranges from 80 to 



s Dr. Slopes and Professor D. M. S. Watson adopt a much lower ratio for the 

 compression. They figure a huge coal-ball which ' has entirely replaced the coal- 

 seam where it occurs, leaving but a film of coal at the top and bottom ' and 

 it is ' nearly 4 feet thick, while the coal on either side is under 1 foot ' [Phil. 

 Trans., B. 200, p. 174). The evidence of this great ball is not at all complete. 

 as not only is there a film of coal of unstated dimensions above and below, but 

 ' streaks of coaly matter run irregularly through it.' Against this may be cited 

 Eenault and Zeiller, quoted by Drs. Stopes and E. V. Wheeler. They measured 

 the tracheids in coal and ' other portions preserved uncrushed as a mineralised 

 petrifact. . - . They concluded that the specimen of wood (of Arthropitvs 

 bistriata) in the coal occupied only one-twelfth of the volume it had in life.' 



