November 1, 1890.] 



KNOWLEDGE 



255 



extent, and 2,296 feet thick, and lies in a depression in the 

 older rocks. One of its most notable characteristics "is 

 the large proportion of coarse materials — 80 per cent, of 

 the strata are conglomerates, and similar rocks ; in our 

 own coal-measures conglomerates are the exception. We 

 have also to note the want of parallelism among the beds ; 

 the great variation in nature and thickness of individual 

 strata ; that beds and sets of beds frequently disappear ; 

 the frequent occurrence of false bedding ; that fragments 

 of granite, sandstone, and other rocks are found in the 

 finer grained deposits ; the black sandstones which contain 

 coal in all dimensions, from microscopic particles to masses 

 and beds several yards in thickness. 



The cliief coal seam is known as l<i (jrande Couche, of 

 which a detailed description is given. It is chiefly remark- 

 able for the great changes in thickness and composition 

 which it undergoes. When followed from point to point 

 it is found to change from an ordinary bituminous coal 

 to canuel, boghead, bituminous shale, and even to sand- 

 stone and conglomerate. In thickness it varies from to 

 upwards of 90 ft. Sometimes the seam is pure coal from 

 the floor to the roof, with a thickness of from 32 to 75 ft. ; 

 sometimes there are intercalations of shale, and even 

 sandstone and conglomerate, which may reach a thickness 

 of 26 ft. No fire-damp is disengaged from this seam, in 

 which it differs from some of the other seams of the basin, 

 but carbonic acid gas occurs. Towards the west it splits 

 up into six different seams. 



In our own country the enormous 30-feet seam of the 

 Dudley coal-field breaks up into ten or fourteen different 

 seams, and in the coal-measures of Northumberland and 

 Durham it has been noted that most of the seams, when 

 followed far enough, coalesce with some other seam. The 

 roof and floor of the seam are variable both as to nature and 

 form. The former is often of shale, and sometimes contains 

 blocks of granite and quartz : it is frequently irregular, 

 and contains fragments of coal mingled with the shale. 



Below, the line of junction is also irregular, the coal 

 frequently denticulating and digitating with the bed 

 below. In many places, also, the coal has been eroded. 

 It was the constant presence below the coal of fire-clay 

 full of stigmaria in the Welsh coal-fields which led Sir 

 Wm. Logan to the conclusion that these clays were 

 ancient soils, and to the general adoption of the theory of 

 growth in situ. 



But, says M. Fayol, in the whole coal-field of Commen- 

 try there is not a single bed which resembles an ancient 

 soil. The (irande Courlw rests sometimes upon carbon- 

 aceous shale, sometimes upon sandstone, sometimes on 

 conglomerate : in no part of the floor are there to be 

 found traces of atmospheric action. All these details of 

 the seam are illustrated by plans and sections. 



M. Fayol takes us to precarboniferous times, and restores, 

 from geological evidence, the physical features of the dis- 

 trict when the deposition of the coal-strata began. The 

 coal-measures lie in a depression in the older rocks which 

 consist of gneiss, granite, grauulite, mica-schist, micro- 

 granulite, porphyries, Ac. A caretul study of the com- 

 position of these is a needful prelimmary to the study of 

 the lithology of the coal-measures. 



The laborious method of percentages employed by RI. 

 Fayol in this part of his work is one which has yielded 

 many valuable results in various branches of geological 

 research. Here it leads to the division of the coal-field 

 of C'ommentry into several distinct lithological zones, 

 shows the origin of the sediments, and enables us to 

 follow step by step the formation of the strata. The 

 method is as follows : — 



Every pebble in a portion of a given bed is talan, and 



its nature, form, and size are noted. Then the pei'centage 

 of eacli kind of rock at many difierent points is calcu- 

 lated. From this work it soon appears that these pebbles 

 are identical with the pre\iously studied rocks of the older 

 formations. Moreover, the percentage of a certain rock is 

 found to increase in one direction and deci-ease in another. 

 This indicates at once the direction of the source from 

 which that particular rock was derived, and the previous 

 study of the surrounding district points to the place of 

 the parent rock. 



" I have found m the neighbouring mountains," says 

 M. Fayol, " the origin of the greater part of the elements 

 which enter into the formation of the coal-measures." 

 The old rivers which brought the pebbles must have 

 flowed along the Hues thus laid down. The form of the 

 pebbles further indicates the nature of the stream, and so 

 gradually the shadowy outlmes of the past fix themselves : 

 the physical geography and climate of the carboniferous 

 period in Central France are restored. An Alpine district 

 appears. High and steep mountains of granite, granu- 

 lite, and mica-schist surround a number of deep, but not 

 large, lakes, scattered over the central plateau. Rain and 

 atmospheric mfluences are at work, and pebbles, sand, 

 mud, and vegetable matter are carried down into the 

 lakes. Gradually the valleys are deepened, and the lakes 

 filled up by the torrential water-courses from the hills. 

 That of Commentry is fed chiefly by a mountam stream 

 from the north, which enters by tlie valley of Bourrus, 

 and another from the west, which enters by that of 

 Colombier : the water flows out to the south by a single 

 outlet. 



Each stream forms its own delta, and these gi-adually 

 increasing join each other, and finally fill the lake. A 

 series of illustrations show how the deltas of the difierent 

 streams approached each other. 



Vegetable df'hris, and the finer sediments, are deposited 

 in the creek between the two chief deltas, and form the 

 chief coal-seam of Commentry — l<i Ordwl,- Coiulw. 

 Another creek, on the other side of the delta, receives the 

 vegetable ih'hrix which goes to form the coal-seam of 

 Ferrieres. In his experiments on sedimentation M. Fayol 

 has shown that pebbles, sand, mud, and vegetable remains, 

 carried by the same stream, may thus be deposited in 

 separate beds. While these beds were being laid 

 down, a geological incident took place which has left its 

 mark among the rocks of Commentry. A huge landslip 

 occurs among the moimtains : the stream is dammed 

 back, accumulates behind the barrier, and then with a 

 mighty rush breaks through, and carries with it enor- 

 mous masses of rock. Large angular blocks of stone are 

 deposited among the strata "of the lake. It is to be noted 

 that Mr. Godwin-Austin, writing ol these coal-fields of 

 Central France, attributes the large blocks to the action of 

 ice. But, according to M. Fayol, no trace of glaciers has 

 been met with. 



The climate of the period, as indicated by the great 

 variety of the plants and insects, was warm and moist, 

 like certain regions situated near the tropics at the present 

 day. Rains were abundant, but not diluvial or extraordi- 

 nary. That the streams which carried the fragments of 

 granite, gneiss, I'tc, from the mountains, brought also the 

 vegetation of the period to be thus separately deposited to 

 form coal is M. Fayol's most important result. The form 

 of the basin, he points out, and the coarse nature anil lie 

 of the beds, indicate a deep lake surrounded by high 

 moimtains, and fed by mountain torrents: the relations of 

 the coal-seams to tlie other beds precludes the idea of 

 intervals during which the area formed swamps for the 

 growth of coal in situ. 



