538 



THE GEOLOGIST, 



QH THE FAULTS OF THE LANCASHIRE COAL-FIELD. 



By Henry Green, Esq. 



The author proposed to point out a law which appears to govern the direc- 

 tion of the principal lines of fault in the Lancashire coalfield, and to endeavour 

 to show, on the principles laid down by Mr. Hopkins, that this law is a neces- 

 sary consequence of the forces which produced the upheaval of the coalfield. 

 On the eastern and northern sides the coal-measures are bounded by millstone- 

 grit, which rises conformably far beneath them. The intensity of upheaval 

 along the eastern boundary was certainly great, as might be expected from its 

 proximity to the central upheaval of England, and there seems reason to believe 

 that it increased in magnitude northwards. The force of elevation along the 

 northern boundary seems also to have increased towards the east ; since it 

 would appear that the north-eastern was a point of maximum elevation. On 

 the south, the coal-measures pass regularly but unconformably beneath the 

 Permian and New Red Sandstone formations, the boundary-line being deeply 

 indented by faults, along which promontories of New Red Sandstone run up 

 into the heart of the coalfield. That the portion lyin<? between the Upholland 

 and Boundary faults contains the same measures, and has been acted upon by 

 the same forces of elevation as the main body of the coalfield, cannot be 

 djoubted; but its position without the basin, and some irregularity in the 

 directions of its lines of fault, lead the author to think that local causes have 

 chiefly determined the arrangement of the measures. It would appear that 

 t e elevating forces have acted with greatest intensity along the northerly and 

 Cpsterly boundaries, increasing in each case towards the north-east corner. It 

 appeared also that the western boundary has been a line of upheaval of smaller 

 a d more uniform intensity, and that towards the south the amount of elevation 

 h s decreased to a minimum. The upheaval-area may be roughly supposed 

 to be oblong in shape, its longer axis running in an east and west direction, 

 and while its southern and western sides remained fixed, its north-east corner 

 was elevated in a vertical direction. As to the extension of lines running 

 north and south across the area, it is evident that it will increase as we recede 

 from the western side ; in fact, it varies very nearly as the square of the dis- 

 tance from that side. In the same way the tension of a line running east and 

 west will vary very nearly as the square of its distance from the southern 

 boundary. Thus, over the uplifted area there will be two sets of parallel ten- 

 sions, the one acting in a north and south direction nearly, and increasing in 

 magnitude from west to east ; and the other in an east and west direction nearly, 

 and increasing in magnitude from south to north. The alteration in the shape 

 of the area produced by its extension will make the lines of tension deviate a 

 little from a northerly and eesterly direction, so that the angle between them 

 will never be quite a right angle. The author had applied Mr. Hopkins's cal- 

 culations to the present case, and obtained the following results with regard to 

 the direction of the first formed set of fissures : — 1. When the two tensions 

 are equal, a fissure will tend to be formed in a direction at right angles to the 

 line bisecting the angle between them. 2. When the tensions are unequal, in 

 \\ hich the tendency to form a fissure is greatest, makes a larger angle, with the 

 direction of the greater tension than with that of the other, this angle tends 

 to a right angle as its bisecting valve. Now, since one tension depends only 

 on the distance from the western boundary of the area, and the other on the 

 distance from the southern boundary, the tension will be equal when these dis- 

 tances are equal, Ilrnee, in every part on this line the fissures will tend to 



