206 



D. Jones — Denudation of Coalhrooh-dale. 



this subject, except for the purpose of showing that the Coal-field of 

 Coalbrook-dale once extended far to the south, but was afterwards 

 cut away by denudation to its present limits. 



Taking this view of the matter, we can now readily understand 

 how it is that the Spirorbis Limestone of the upper Coal-measures is 

 found in the south of the field at 170 feet above the base of the older 

 Coal-measures. The greater part of those older strata have been 

 washed away, and the younger Coal-measures, with the Spirorbis 

 Limestone, have been subsequently deposited upon the remaining re- 

 duced thickness of the old strata. When therefore Mr. Prestwich 

 gives the total thickness of the Coal-measures of this field at 750 feet, 

 as shown in the Madeley Meadow Pit, it should be added that a 

 middle slice of about 250 feet is wanting in that .shaft. Thus it 

 appears that the denudation did not affect the eastern boundary, only 

 producing what is now known as the Synion fault of Mr. Marcus 

 Scott ; but it affected the whole of the southern part of the Coal-field 

 from a line drawn between Ketley and Prior's Lee. The plan, with 

 its lines of denudation shown at several vertical ranges, is only given to 

 illustrate my view, and those lines must only be taken as approximate. 

 There is one more point to which I must advert. Mr. Eandall 

 says, in his ninth letter to the Mining Journal, " The valley of denuda- 

 tion, however, does not appear to have been filled up by these younger 

 members of the Coal-measure series, hence the Permains came up and 

 overlap the whole along an undulating line running north and south 

 parallel with that of the Symon fault." And again, " It is remark- 

 ably instructive as regards the nature of this fault that just on the 

 line where the Coal-seams terminate the Permians make their appear- 

 ance, rapidly increasing in thickness as one after the other of the 

 former disappear." (Letter ix.) The following section would repre- 

 sent what Mr. Eandall means : — 



Fig. 1. Valley of Denudation, according' to Mr. Randall. 



O. Older Coal-Measures. 



+ Line of erosion. 



Now the fact is that the younger Coal-measures not only fill up 

 the eroded valley, but overlap the older Coal-measures, and the 

 Permian overlie the younger formation, thus — 



Fig. 2. Actual Valley of Denudation Coal-Measures. 



Permian Rocks. 



O. Cider Coal-Measures. 



y. Younger Coal-Measures. 



