1 1 2 Rev. W. D. Conybeare aii the Structure and Extent 



has, therefore, the effect of rendering this important part of 

 our deposits accessible throughout a much greater extent than 

 would otherwise have been the case. Martin's view of the 

 section of the basin beins thus : — 



Martin's Structure. 



North 



South, 



The true Structure. 



Xorth. 



South. 



I shall call the two subdivisions of the great basin, hence re- 

 sulting, A and B, taking A for the northern division, B for 

 the southern: the strata lie much more flatly in A than in 

 B, the dip in the latter being always considerable. 



Before I proceed as to the extent of the beds, I would men- 

 tion, that it appears to me that our calculation ought only to 

 include the workable extent. Now I can hardly consider coal 

 beds as workable at a lower depth than 200 fathoms beneath the 

 lowest points of the adjacent valleys. Beyond this the expense 

 of drainage, &c. &c. would be too enormous ; I therefore do 

 not think we can reckon on the whole extent of any given bed 

 in the coal-field, but only on that part of its extent which is 

 within this depth from the level of the adjoining valley. 



I find it convenient, for the purpose of our examination, 

 to divide our coal-basin into four districts: 1. from Pontypool 

 to the valley of the Taafe ; 2. thence to Dyffryn Llynfi ; 

 3. thence to Kidwelly ; 4. the Pembrokeshire portion. 



The principal part of the range of my anticlinal line is in 

 District II. though its extremities extend into District I. on 

 the east, and into District III. on the west, but not so as ma- 

 terially to affect our calculations. 



District I. — The outer edge of the lower series ranges 

 along a line of 35 miles, from Merthyr, by Pontypool, to the 

 gorge of the Taafe at Castle Coch, three miles north of Cardiff: 

 when I say the outer edge, I do not mean the extreme 

 boundary, viz. the outcrop of the very lowest coal bed; be- 

 cause, if I took it so, I should of course assume an extent too 

 great for the upper members. I have taken, therefore, a mid- 

 dle 



