142 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 5, 



of sediment in the lower portion, and a decrease in the upper, as 

 compared with the northern districts of England. 



The following seems to be the corresponding series in both countries : 



Carboniferous Series of England and Scotland. 



England (Lancashire). Scotland (Lothians). 



Feet. Feet. 



1. Upper Coal-measures 2,000 1. (Lost by denudation ?) 



2. Middle Coal-measures 3,200 2. (Partially denuded ?) 1000 



3. Lower Coal-measiires 2,000 3. (Supposed to be absent) 



4. Millstone-gi-it 3,000 4. Eoslyn Sandstone Group ... 1 500 



5. Yoredale Rocks 2,000 5. Edge-coal Group 900 



6. Limestone (no "sedimentary" 6. Lower Carboniferous series ] 



strata) 2,000 (shales and sandstones, \ 3000 



with httle limestone) 



Total "sedimentary" strata. . . 12,200 Total " sedimentary" strata. . . 6400 



It will thus be observed that, even allowing 2000 feet for the 

 upper portion of the Scotch series, lost by denudation, the amount of 

 ^' sedimentary " strata in Scotland could not reach that of Lancashire, 

 notwithstanding the accession it receives in the horizon of the Car- 

 boniferous Limestone. Future investigation will probably result in 

 adding considerably to the thickness of strata, and in throwing some 

 light on the equivalents of the Gannister Beds, which in the north 

 of England form a most interesting and important group of strata*. 



Two other suppositions, however, may be advanced, one of which 

 is that we have here a case of compensation not unfrequently to be 

 observed ; and that, as the sedimentary strata have received so large 

 an accession in the lowest member of the series, they have had a 

 corresponding reduction in the upper portions of the system, as com- 

 pared with England. But the supposition which I regard as the 

 more probable is, that we may here have passed across the position of 

 maximum accumulation, and may have reached the point where the 

 beds begin to thin away in the direction of the old coast- line, as 

 represented in fig. 4, page 135 1- 



2. Region South of the Barriee. 



We must now retrace our steps to the district south of the barrier, 

 which includes the coal-fields of South Wales, Forest of Dean, Bristol 

 and Somerset — Mr. Godwin- Austen's hypothetical trough of the 

 Thames Valley, and the culm series of Devonshire. 



The sedimentary strata of this region appear to have been derived 

 not from the north-west, as in the case of the coal-fields north of the 

 barrier, but from the west- south-west, as indicated by the isometric 



* Mr. Geikie has suggested to the author, as a possible explanation of the 

 absence of the Gannister Beds or Lower Coal-measures of England, that the 

 Scottish area was elevated into land during the period in question. 



t Taking the line A as the point of maximum accumulation, this may represent 

 the Carboniferous series of Lancashire and Yorkshire, while the Scottish Coal- 

 fields will be situated at h, and the EngHsh atB, Q, D, E, — E being the vanishing 

 point towards the South-east of England. 



