LESSONS IN GEOLOGY. 



197 





ban generally obliterated all traces of vegetable conn traction in 

 the coal ; however, careful search is sometimes rewarded with 

 a thin Hootion of a piece of ooal which exhibiU the fibres of the 

 woody growth, and BO proclaim* the species to which it belonged. 

 Ily thin inoaiiM all tho specie* of vegetable life discovered as 

 fossils in tho rocks of the period have been detected in the ooal. 

 A moment's consideration will show that we ought not to be 

 surprised that tho ooal of each locality in chiefly composed of 

 trees of the same kind, fur just OH to-day wo find our oak forests, 

 pine forests, mango groves, so then a plantation of Sitfilluriiu 

 wuuM re-sow themselves and gradually extend on every side, and 

 the ooal produced on that site would be the remains of that one 

 class of tree ; of oourso, it would frequently happen that other 

 species would be also found. 



iv strange feeling, as wo walk in a dark coal-pit, to think 

 that once tho black cold stone was a living forest, luxuriating 

 under a warm sun and a moist atmosphere ; that that very sun- 

 light which conduced to tho growth of tho forest is, as it were, 

 fossilised in the ooal, and here, after the lapse of uncounted 

 ages, we are unearthing the light, to brighten our houses and our 

 streets. So ia it that we " require that which is past." 



The floor of every coal-field is indurated stone or shale. This 

 was once the earth of the forest in which tho trees spread their 

 roots, and from which the rootlets gathered in their sap. If 

 minors did not work in tho dark, the marks of the trees would be 

 frequently found. In Staffordshire, some years ago, a seam of 

 ooal waa laid bare upon tho surface, and in a space of a quarter 

 of an aero as many as seventy-three stumps were apparent. 

 Tho trunks wore broken off close to the roots, and the trees were 

 overlying each other, all flattened and converted into coal. The 

 roots rested on a bed of what had been clay, two inches thick ; 

 below this was another forest, and five feet lower down a third. 



The process which hod tended to produce this state of things 

 would seem to be that tho forest had grown, and generations 

 after generations of trees had contributed their quota to the 

 accumulation of matter ; then the land sunk, or hod been 

 constantly and gradually sinking until it was covered with 

 water ; then came tho deposit which formed the clay. Upon 

 tho re-appearing of this above the surface the forest again 

 began to grow, and the second seam of coal is the result of 

 its luxuriance ; and so on with the rest. But here a difficulty 

 presents itself. If these inundations were of such constant 

 occurrence, how is it that the coal is seldom or never intermixed 

 with sand ? For frequently we find a seam of coal as many as 

 thirty feet thick, and what an enormous accumulation of vege- 

 table matter this compressed and solidified mass represents. We 

 can hardly conceive the carboniferous forest growing for such 

 ages in a low- lying swamp, and not being subject to frequent 

 inundations ; and yet where are the signs of such floods ? More- 

 over, wo have frequently absolute proof of tho force of water in 

 the roots of tho Sigillariso, which must have been torn up, and 

 tho water-worn appearance of the vegetable debris ; and yet 

 there is no sign of sand. How the water had failed to carry and 

 leave a, deposit of fine mud was a difficulty which long wanted 

 a satisfactory solution. Observation has at last supplied an 

 answer. The forest swamps in the delta and valley of the 

 Mississippi are much in the supposed position of the carboni- 

 ferous forests, but here wo find the margin so clothed with a dense 

 growth of roods and herbage that the river water ia really filtered 

 as it passes through the jungle, so that no appreciable deposit is 

 made in the interior of the forests where the vegetable accumu- 

 lation is progressing. Lyell mentions a happy confirmation of 

 this suggestion to be observed in tho swamps of Louisiana, that 

 when from long drought a swamp dries up and accidentally 

 catches fire, the fire burns many feet deep until it reaches water; 

 but scarcely any residuum is found at the bottom of the pit, 

 showing that the swamp accumulated vegetable matter without 

 any admixture of a mineral deposit. 



It is worthy of remark that at the bottom of all these cypress 

 swamps a bed of clay is found, in which are the roots of the 

 trees, just as in the underclays of the coal are found Stigmariffl. 



Having given thus a brief summary of the probable mode in 

 which our coal-beds were originated, we pass on to the more 

 immediate history of the great mineral deposit. 



The wealth of the period will be sufficiently set forth if we 

 confine our remarks to the British Isles alone, without taking 

 into consideration the coal-fields of other countries. In these 

 islands there are more than 3,000 mines, which arc mainly 



situated in the northern counties. Tb*ir yearly produce is 

 more than 100,000,000 tons, and when we consider that the 

 average price of a ton of ooal in the London market is nearly 

 twenty shillings, we may gather some idea of the enormous 

 source of weeJth preserved for ns in the carboniferous system. 



The proportions in which this ooal is scattered to its various 

 destinations may be learnt from the accompanying table, which 

 gives the quantities the mines of Northumberland sad Durham 

 yielded in one year, and the mode in which it was copsnmed : 



Coal exported to torttgn eeeatries . 

 Coke oostjmted as eosl . 

 Coal sent coast-ways . . . 



Coke computed u 00*1 . 

 Coil carried away by rail . 



Coke computed M ooal . 

 Collier/ consumption . . 



Consumption in the district and In the district 

 manufacture* ..... 



4.910.OT 

 ejss7 



i/m,4S7 



1,850.000 

 2,975,000 



Total . w.wr.m 



No small quantity of ooal is taken from our shores, aad fires ia 

 cold Lapland as well as the hearths of the people of the Sooth 

 Sea Islands burn cheerily with our biasing British coal. Coke 

 remains in the retorts after ooal has undergone what the chemist 

 terms "destructive distillation ;" that is, when ooal is snoloead 

 in iron chambers which have but one exit, and these are heated 

 in furnaces. Being deprived of air the necessary of combustion 

 tho coal cannot burn, but is caused by the heat to decompose, 

 giving off two volatile pro-tacts one a mixture of gases, which 

 when purified, we burn in our houses ; the other is a liquid known 

 as ooal or gas tar. From this liquid many useful products have 

 been obtained, especially the aniline dyes, the brilliant magenta, 

 and the soft mauve. Coke, therefore, is that part of coal which 

 is not volatile, and is used whenever a smokeless fuel is required, 

 for everything which it was possible to drive off by beat having 

 already been dissipated, nothing which could produce vapours or 

 smoke remains. 



Extracting coal from the bowels of the earth gives employ- 

 ment to a large portion of the population living in the ooal 

 districts. As may be gathered from the repeated colliery explo- 

 sions, the undertaking is not without danger. Add to this the 

 dark dismal labour, hewing the black stone in narrow galleries, 

 with a feeble light, the solitary nature of the work, and the long 

 hours spent from the light of day nay, many miners never see 

 daylight, except on Sundays, for they descend the shaft before 

 the dawn, and the sun has set ere they again come to the surface 

 all this makes the miner's calling anything but desirable. 



The general principle upon which coal is mined is this : A shaft 

 or circular hole is sunk down to the seam often even 500 yards 

 deep ; a gallery is then worked, and the coal sent to the surface. 

 It is necessary on account of the gas which fills the rents and 

 chinks in the coal-bed, as well as to give air suitable for the 

 miners to inhale that there should be a current of air passing 

 through the gallery. This is generally effected by sinking two 

 shafts, and connecting them by the underground way ; at the 

 bottom of one shaft a furnace is lit, which causes a current to 

 rash up the shaft, as up a chimney. Thus a current descends the 

 other, or the " down shaft," traverses the gallery, and ascends 

 the "up shaft." As the working of the mine extends, and 

 lateral cut tings are made at right angles to the " main way,' ' doors 

 at their entrance are erected across the path of the current, so 

 as to force it along the side cuttings. Boys sit to open and shut 

 those doors, as the wagons go to and from the foot of the shaft. 

 In large mines the ventilation becomes very complicated, and a 

 very strong current of air is required to traverse the whole of 

 tho tortuous windings of the mine. When the seam lies very 

 deep below the surface, so that the sinking of the shaft is a very 

 expensive work, then the shaft is divided by a light wall or 

 brattice, so that the air is made to descend one side, while the 

 hot current ascends the other, drawing with it the foul air of the 

 mine. 



Tho fire-damp, a cause of such destruction, is a mixture of 

 gaseous hydro-carbon, and all but identical with the gas we burn ; 

 indeed, in many mines the main ways are lit by pipes thrust into 

 the crevices of the ooal, or the gas as it issues frern the rents in 

 the seam which it often does in such quantities as to doseivn the 

 name of " blowers " given to it by the miners is conducted to 

 a gasometer, and there reserved for the use ot the <-' 



