172 Chemistry. [Lecture 30 



which there is no particular rule ; then perhaps 

 to a bed of freestone ; and, lastly, to the coaL 

 After they have wrought the coal, they are ob- 

 liged to sink another pit ; and in this they find 

 the same strata varying in thickness as above. 

 These extensive bodies are found most regular 

 when the country is flat, being generally parallel 

 to the horizon, though frequently dipping down- 

 wards in a certain angle. In many places the 

 beds have a wave, as where the country consists 

 of gently rising hills and vales : here too they 

 generally dip. In riding a mile, we perhaps 

 pass through ground composed mostly of sand, 

 in another mile, perhaps of clay ; and this is oc- 

 casioned by the edges of the different strata lying 

 with an obliquity to the horizon. The projection 

 of the edges of other strata is called by the mi- 

 ners cropping, and where the edge shows itself, 

 the crop. By the same kind of projection, moun- 

 tains, or ridges of mountains, are produced, and 

 the form of them regulated. Mountains are said 

 to have a back and a face ; the latter smoother, 

 and the former more rugged. We generally find 

 too on one side of the mountain a more gradual 

 ascent than on the other, which is occasioned by 

 the cropping of the strata. The back of the 

 mountain shows the obliquity with which the 

 strata sinks into the ground ; the abrupt edge of 

 the strata, however, becomes more sloping as 

 time, producing a decay, draws rubbish from 

 above. Where the face of the country is irregu- 

 lar, this appearance depends on the different 



