318 



M I N E. 



Mines of 



Singular 

 coal-field. 



SECTION of the Coal Strata at JOHNSTON, in the County 

 of RENFREW. 



No. 



Depth or 

 coal strata. 



Names of the Strati. 



1. Greenstone 



2. Sandstone and common indurated clay, alter- 



nating in thin bands . 



3. Fire-clay, with coarse ironstone 



4. Coal 



6. Indurated clay 

 G. Coal 



7. Indurated clay 



8. Coal 



9. Indurated clay 



10. Coal 



1 1. Indurated clay 



12. Coal 



13. Indurated clay 

 II. Coal 



15. Indurated clay 



16. Coal 



Yards. Feet. Inch. 



36 







81 



The above is a very interesting section of a coal- 

 field, which, according to Werner, belongs to the New- 

 est Floelz trap formation. The striking peculiarities 

 are, 



1st, The great body of greenstone of the common 

 crystallized texture, known in Scotland by the name 

 of blue whinstone, found at the surface, and lying above 

 the common coal 'strata, which are comparatively soft, 

 and have little coherence. 



2d, The vast body of coal lying together, consisting 

 of 10 beds. There are only 7 beds in the section, but 

 the fourth coal is commonly reckoned three beds, and 

 the lower coal two beds, there being a difference in the 

 quality, with thin divisions in some places betwixt 

 them. The whole thickness is 90 feet 2 inches. Some 

 of the coal is of the open burning kind, but the great 

 part is of the close burning quality, similar to New- 

 castle coal, and breaks into small pieces. The coal 

 abounds with inflammable air, and is liable to sponta- 

 neous ignition. In a great part of this coal-field the 

 coals amount to only one-half of the thickness repre- 

 sented in the section ; but in the place where the sec- 

 tion is taken, the coals lie as if they had been cut 

 through, and one-half slid over the top of the other. 

 This singular coal-field is very limited in point of ex- 

 tent. 



From an inspection of the four preceding sections, 

 it will be seen, that, although the alternations of the 

 strata are various, they may be referred to two kinds ; 

 viz. sandstone and clay variously modified ; the only 

 exception is the greenstone. There is no bed of lime- 

 stone in any of them ; this is however one of the strata 

 found in coal-fields, and sometimes forms the roof of a 

 coal. Beds of limestone are generally found in the low- 

 est series of the coal strata of a district. 



With regard to coals termed workable or unwork- 

 able, this does not depend upon the thickness of a bed 

 of coal, but upon the price obtained for it; so that a coal 

 which is unworkable in one district, on account of 

 its thinness, would be workable to advantage in 

 another. 



The greatest depth to which coal strata have been 

 ascertained is 500 fathoms. This has been done in 

 the Newcastle district, but it is not supposed that they 

 go down to an indefinite depth, from the bason shape of 

 coal-fields. It is with reason supposed, that the rocks 

 upon which the Northumberland coal-field rests are a 



continuation of the metalliferous beds which form moun- 

 tains towards the county of Cumberland. 



No coal-fields of any extent are found to be absolute- 

 ly flat, although small portions of them may be so ; they 

 generally lie with an angle of inclination to the horizon ; 

 they are, however, found at every Various angle of in- 

 clination ; viz. from the horizontal to the perpendicular 

 line ; but the most common range of inclination is be- 

 twixt 3 and 1 1 5 degrees ; that is, from a rise of one foot 

 perpendicular in twenty feet horizontal, to one foot 

 perpendicular in five feet horizontal. In coal mining 

 the angle of inclination is seldom mentioned, the com- 

 mon term for the inclination, or dip, being stated as 

 one in five, one in ten, or one in twenty, according to 

 the circumstances of the case. A coal is reckoned very 

 flat which lies at an inclination of about one in twenty ; 

 and a coal is reckoned to have rather a great inclina- 

 tion, or what is termed a quick dip, when it lies at an 

 inclination of about one in four. Coals having an in- 

 clination of from one in three, to one in one, are termed 

 half edge coals ; and those which have a greater inclina- 

 tion than one in one, are denominated edge coals. 



As coals with their concomitant strata lie in a con- 

 forming situation, and parallel to each other, similar to 

 the leaves of a book, and at an inclination with the ho- 

 rizon, having a cover of earthy alluvial matter in ge- 

 neral over them of considerable thickness, each of the 

 strata meets the alluvial cover in the manner as repre- 

 tented, Plate CCCLXXXV1II. Fig. 1. where A is the 

 surface line, B the under part of the alluvial cover C, 

 and a, a, a, coals with their accompanying strata. This 

 alluvial cover is found to be of various thickness, from 

 a few inches to many fathoms ; and it is this covering 

 which creates the greatest difficulty in searching for 

 coal and other minerals. It is in mineralogy to the strata 

 which are underneath, what the flesh is to the bones in 

 anatomy, when the covering is removed, the absolute 

 structure is at once seen. 



In describing, the absolute forms or shapes of coal- 

 fields, it is necessary that the cover of earth above the 

 strata be considered as removed, that every point may 

 be more easily comprehended ; and, to render the sub- 

 ject less intricate at first, we shall consider the coal 

 fields as being of a regular shape, and without disloca- 

 tions. 



In general, coal-fields have either the form of a cir- 

 cular or long elliptical bason, or are segments of these 

 figures ; so that no extensive field of coal lies as to itself 

 in the same plane, though a small portion of it may ap- 

 pear to be so. There are but few instances of the entire 

 bason form, but this form we shall first explain, in or- 

 der to lead on to the more intricate varieties which oc- 

 cur. Plate CCCLXXXVIII, Fig. 2. represents the ho- 

 rizontal sectitfn of an elliptical coal-field, with the coals 

 A and B rising to the surface in every direction, with 

 their accompanying strata, and dipping to the centre 

 of the bason C. Fig. 3. is a section of the bason in the line 

 DE. It is evident from an inspection of the figures that 

 the strata in the lineFG,(Fig.2.)have a greater dip than 

 the strata in the line DE, because they reach the cen- 

 tre of the bason in a shorter horizontal distance ; hence, 

 if the bason were forty times longer than it is broad, 

 the dip of the strata from D to C, and from E to C, 

 would be very flat when compared with the dip from 

 F to C, and from G to C, which would be comparative- 

 ly great. Here we conceive the bason to be of a regu- 

 lar elliptic form, but we find in general that the trans- 

 verse line DE does not divide the bason equally, but 

 unequally, as represented in Fig. 4. where it is evident 



Mines of 

 Coal. 



Angle or 

 dip of the 

 strata with 

 the horizon. 



PLATE 



CCCLXXXTIII. 

 Fig. 1. 



Forms or 



shapes of 

 coal-fields- 



Fig. 2. 



Fig. 3. 



Fig. 4. 



