483 



geologists are now exploring several parts of it with the feeHng 

 that they are entering upon terra incognita. Not only do they find 

 it necessary to trace out more correctly the limits of formations 

 previously known, but also to introduce new groups of fossiliferous 

 strata and new divisions, in districts before supposed to have been 

 well investigated. 



The carboniferous deposits which are alike interesting, in a scien- 

 tific and economical view, have deservedly occupied of late the par- 

 ticular attention of many able geologists, and we have received com- 

 munications on the subject from Mr. Murchison, Mr. Prestwich, 

 Professor Sedgwick, and Mr. Peile. The observations of Mr. 

 Prestwich relate to the coal-measures of Coalbrook Dale, and the 

 formations immediately above and below them, together with the 

 accompanying trap-rocks. 



There is perhaps no coal-field in the whole country of equal size 

 in which the strata have been so much dislocated and shattered. 

 Mr. Prestwich gives a detailed description both of the principal 

 and minor faults, their direction, extent, inclination, breadth, and 

 fall, and the difference of level produced by them in their opposite 

 sides, which is sometimes slight, but sometimes amounts to 600 or 

 700 feet. In some instances the change of level is by steps or 

 hitches, which, it is truly said, may be owing either to unequal re- 

 sistance, or to a series of small dislocations. The walls of the 

 fissures in the disjointed strata are sometimes several yards apart, 

 the interval being filled with the debris of the strata. In other 

 places they are in contact. In this last case it is particularly re- 

 marked that the surface of the ends of the fractured beds of coal 

 and shale is shining and striated. You are aware that this appear- 

 ance has usually been attributed, and I believe rightly, to the 

 rubbing of the walls of the rent one against the other, the lines of 

 the polished and striated surfaces indicating the direction of the 

 motion, but I have lately seen it objected to this theory, that the striae 

 are not always parallel, but often curved and irregular, and that 

 the earthy contents of veins and faults often present the same glit- 

 tering and striated faces, or slickensides as they have been called. I 

 am familiar with the fact, and have always inferred that the movements 

 were irregular and complicated, occasionally changing their direction, 

 and that even when unifonn, tliey may have acted u"nequal!y on mate- 



