MODE III. COAL. 577 



pebbles enveloped in ferruginous sand." * He 

 afterwards observes, that the limestone which 

 contains coal, often becomes black from inhala- 

 tions of the bitumen, while the inherent shells 

 are of a white colour. 



A remarkable circumstance in coal mines is sups or dykes, 

 the frequent occurrence of what our miners call 

 slips or dykes, while the French call them crelns 

 or failles, consisting of indurated clay, basaltin, 

 called whin in Scotland, and sometimes of sand- 

 stone. These are sometimes of great extent, and 

 a whin-dyke is said to pass across the estuary of 

 the Forth, from East Lothian to Fife, a distance 

 of 10 or 12 miles. These slips intersect the 

 strata of coal, almost at right angles; and ge-- 

 nerally derange them, in regard to elevation, the 

 stratum of coal being higher and lower on the 

 different sides of this interruption. It seems a 

 general observation that the strata always sink, 

 on what may be called the back of the slip, 

 which seems to indicate that the matter was 

 ejected from beneath, and that the consequent 

 cavity had occasioned the subsidence on that 

 side. These slips sometimes contain fragments 

 of coal f. They appear in the section of the 



4 



Min. ii. 6. 



f Near the slips the quality of the coal changes ; it sometimes 

 becomes iridescent j still nearer it splits and is friable 5 then be- 



YOL. I. 2 P 



