MILLSTONE GRIT. 



117 



reduced to seven feet. There is also a remarkable and apparently sudden change in 

 the mineralogical structure of a portion of the beds of the great coal and of the Smith 

 coal, lying between the Thirty-six feet and High faults ; by the interpolation of layers 

 of pure Cannel coal, which alternate with the ordinary coal. The great coal having in 

 this part a thickness of ten feet is divided in the following manner : — 



Ft. In. 



1. Roofs 3 0 



2. The Bat, a pure Cannel Coal, used in turning for ornamental purposes . 0 10 



3. The Middles 4 0 



4. Cannel Coal, less perfect than No. 2 1 0 



5. The Holeing Coal 0 6 



6. The Bottoms 0 8 



10 0 



The Smith coal contains a layer of three or four inches of Cannel coal, and the Three-quarter 

 coal offers some indication of the same structure. There is not a trace of Cannel coal in any of 

 these beds on either side of the mass included between these two faults. In stating this fact, I 

 would also remark that in the two main faults the Cannel coal and the High fault trend in the same 

 direction as the great Jewstone or Basaltic dyke, which will hereafter be shown to occupy a fis- 

 sure of eruption 1 . 



Besides those enumerated, there are many minor faults, some of which are mere emanations 

 from one or other of the two great dislocations, and have no very determinate direction. The coal 

 of the Knowlbury field having been in great part extracted, the occupier, Mr. Lewis, has recently 

 made several trials beneath the south-western and north-western faces of the great plateau of 

 basalt, which extends from the Cornbrook works. In the trials on the north-western escarpment 

 of this basaltic cover, called the Hoar Edge, he has sunk two shafts, an account of which will be 

 given in the next chapter, when we endeavour to explain the relations of the basaltic rocks and the 

 mode in which they have been erupted. 



Millstone Grit. 



This formation, consisting of pebbly, quartzose conglomerate, and thick-bedded hard 

 white sandstone, rises, at many points, from beneath the productive coal-field. It is 

 most expanded in the sterile tract which lies between the northern slopes of the basaltic 

 hills, and the zone of Carboniferous limestone at Oreton. (PL 30. fig. 1 and 6.) When 

 that limestone is wanting, the Millstone Grit rests immediately upon the Old Red Sand- 

 stone, as may be well seen in the quarries of the southern foot of Titterstone Clee Hill, 

 and beneath the Horse Ditch pits. At this point it passes from a conglomerate into a 

 coarse-grained yellowish and white sandstone of good quality and is extensively worked. 



The Cornbrook field is in great measure drained by a large adit called the Cornbrook level, 



1 These faults vary in width from a few inches to forty-five feet, and they are usually filled with broken 

 coal measures, ironstone, &c. In the space between the overlying basalt of the Hoar Edge, and the productive 

 coal-field of Knowlbury, it is said that all the coal formerly extracted lay in pools or broken masses. 



P 



