of the Forest of Dean Coal-Jield. 205 



somewhat further, though not in a completely straight line ; for near the west 

 end of Ailberton, where the strata dipping with those at Ailberton Common, 

 join the beds which dip conformably to the Blakeney range, a fault beo-ins, 

 separating these two dips from one another. 



The strata which this fault divides, in proceeding northwards, and in which 

 three different dips appear, are, first, old red on both sides ; then, after pass- 

 ing the Chapel, old red on the right, and limestone on the left; in Lydney 

 Park there is limestone on both sides ; then at the back of the Red Hill, the 

 fault divides the millstone grit on the left from limestone on the right. It 

 afterwards makes a little turn to the right, a{)parently just beyond the junction 

 with the lower coal, and cuts across the end of the limestone first, and then of 

 the upper beds of the old red, and of the conglomerate, and some of the beds 

 under it. At the same time the strata to the left of the fault, which are here 

 the coal measures, cltange their direction, and dip thenceforward nearly as 

 those on the right. From that point, therefore, the fault is marked by the 

 omission of the limestone, on the surface, and not by any great difference of 

 dip. 



For some distance, the strata on each side, are nearly parallel to one another 

 and to the fault (see the accompanying diagram) ; but after entering the forest, 

 it is conjectured, for it is difficult to determine from the thickness of the new 

 plantations, that there is a sort of converse in some respects from that which 

 occurs near Lydney ; for while the coal measures {d) continue to range parallel 

 to the fault, the upper beds of the old red («), which had been cut off near 

 Lydney, gradually and obliquely come in again, as inferred from the dip ; 

 and then at Denbigh Lodge the fault turns for a short distance at about 45° 

 to the left, and cuts across the end of the conglomerate (a), the beds above 

 it («), and the limestone (6), which are thus restored to the surface, at the 

 valley of Blackpool Brook. It is presumed that at this spot, the fault returns 

 to its former direction, and does not terminate, for the millstone grit is not 

 restored at this point, though the limestone {h) is, but gradually appears 

 further on (c). 



a. Old red sandstone, a. Old red conglomerate, b. Carboniferous limestone. 

 c. Millstone grit. d. Coal measures. 



Though in one sense, the fault which exists near Ailberton may thus be 



