320 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 8, 



This plan of relative distribution was illustrated on the map of 

 the country by a series of lines of equal thickness (or isometric 

 lines), which, I venture to think, renders the arrangement of the 

 two sets of strata very simple and intelligible. 



Confining my observations to the district north of the central 

 " barrier " of Silurian rocks, and taking, as examples of the south- 

 easterly attenuation (or thinning out) of the sedimentary beds, the 

 cases of Leicestershire and South Lancashire as ascertained by the 

 carefully measured sections of the Geological Survey, I gave the 

 foUowing results:— ^ Thickness. 



Leicestershire 3100 feet. 



South Lancashire 12800 „ 



This augmentation of the same beds in Lancashire, as compared 

 with Leicestershire, appeared sufficiently striking ; but subsequent 

 investigations, while engaged in the survey of the district further 

 north along the Pendle Eange and in the neighbourhood of Burnley 

 and Blackburn, have shown me that it falls short of the full measure 

 of increase by several thousand feet of strata. 



For some years past the geological surveyors have been autho- 

 rized to trace each separate bed of gritstone or conglomerate in the 

 Millstone and Yoredale series ; and these are represented in the pub- 

 lished maps and sections by distinctive tints and patterns. By such 

 means alone could a true knowledge and representation of the 

 structure of the Lower Carboniferous rocks be arrived at ; and it 

 is only after having, with my colleague Mr. Tiddeman, traced these 

 beds through many miles of country, and ascertained their true 

 relations to each other, and their relative and absolute thickness by 

 several comparative sections, that I venture to give publicity to the 

 results they point to. The details, however, of these sections are 

 primarily the property of the Geological Survey, and are to appear 

 in due course in the Memoirs; and this being so, I must claim 

 some amount of indulgence during the interval, and offer only the 

 gross aggregate, as it were, preparatory to a future statement of the 

 individual thickness of the beds. I can venture to assert, however, 

 that the statement I now make will not be found far from the truth 

 when the balance-sheet is presented. 



I have made three transverse sections for the purpose of ascer- 

 taining the thickness pf the Millstone-grit series, with the following 

 results : — 



Sections of the Millstone- Grit sei^ies, Pendle Range. 



Locality. Thickness. Mean 



1. Sabden, near Burnley 6500 ] thickness. 



2. WhaUey Nab 5000 I 5500 feet. 



3. Snodworth, near Blackburn 5000 J 



As the Sabden section is so much greater than the other two, I 

 do not feel so much confidence in it ; but if there is a real exagge- 

 ration, it is reduced by taking the mean of the three thicknesses. 



The results above stated will not appear unreasonable when it is 



