452 J. Prestwich, Esq., on the 



of the sea, and it ranges parallel with the Wrekin. Like that mountain it 

 consists of greenstone, felspar porphyry, and hornstone. I cannot state ac- 

 curately its effects upon the adjacent rocks, but it appears to have brought 

 to day, on its eastern flank, patches of the coal-measures, in the same manner 

 as at Longwood. 



The minerals found in these igneous rocks are confined to small reddish 

 crystals of felspar in the eurite of Ercal Hill ; small crystals of quartz in the 

 cavities and crevices of the greenstones, to a few veins of carbonate of 

 lime, crystals of augite, grains of olivine and traces of iron pyrites in some 

 of the basalts and greenstones. Phrenite and oxide of iron occur in the 

 amygdaloids. 



LINES OF DISTURBANCE. 

 Few coal-fields have probably been more shattered than that of Coalbrook 

 Dale ; and its numerous as well as extensive faults forcibly illustrate the energy 

 of the disturbing agent. Deposits, several thousand feet thick, and occupying 

 an area of many miles, have been shivered into fragments, and the broken 

 remnants have been placed in the most discordant positions, and very often 

 at levels differing from each other, many hundred feet. 



In selecting the faults described in the following pages, I have chosen only 

 such as illustrate the leading effects of the protrusion of the carboniferous and 

 older rocks through the new red sandstone, and which exemplify the magni- 

 tude and direction of the disturbing power. I have prepared a tabular list of 

 the effects of these major faults, together with those of numerous minor 

 ones, Avhich it would be tedious to notice in detail (See Appendix D.) 



The collier can rarely proceed many hundred yards in any direction with- 

 out encountering a fault of some magnitude; whilst slips or dislocations of less 

 importance frequently occur every three or four yards. Although the conti- 

 nuity of the strata is thus interrupted, and some inconvenience is produced; 

 yet the faults are accompanied, as many writers have stated in describing other 

 coal-fields, by advantages which far outweigh the evils arising from their tem- 

 porary obstructions. Various disjointed beds of coal and ironstone are thus 

 brought on a level, though, if their horizontal range had not been disturbed, 

 they would be separated vertically many yards. An instance occurs at Broseley 

 of four beds of coal and one of ironstone, each separated by several feet of 

 shale and sandstone, being rendered accessible upon the same adit by means 

 of successive dislocations. Again, were it not for these fissures, a great por- 

 tion of the deeper part of the coal-field would be unworkable, on account of 

 the water draining from the higher strata. Through their agency also, the 



