241 



from 18 to 20 miles; but in Pembrokeshire its breadth is only from 

 3 to 5 miles. 



On the northern half of the basin the strata rise gradually north- 

 ward ; on the south side they rise southward, except at the east end, 

 where they rise eastward. The deepest part of the basin is between 

 Neath, in Glamorganshire, and Llanelly in Carmarthenshire, where 

 the depth of the principal strata of coal and iron ore is from 600 to 

 700 fathoms ; whereas in Pembrokeshire, none of the strata lie above 

 80 or 100 fathoms deep. 



The strata of coal at the east end of the basin and on the north 

 side, are chiefly of a cokeing quality ; but they alter, towards St. 

 Bride's Bay, to what is called stone coal : on the south side of the 

 basin the strata are principally of a bituminous or binding quality. 



In this mineral basin there are 12 veins, or strata of coal, from 3 

 to 9 feet thick ; and 1 1 others, from 18 inches to 3 feet, making in 

 all 95 feet, besides a number of smaller veins, from 6 to 18 inches in 

 thickness. 



There are in these strata many faults or irregularities, by which 

 the due range of the strata is thrown out of course. These faults 

 are not confined to the edges of the strata, but run through the in- 

 terior of the basin generally, in a north and south direction, and often 

 throw the whole of the strata, for hundreds of acres together, 40, 60, 

 80, or 100 fathoms up or down. There is, however, seldom any su- 

 perficial appearance that indicates a disjunction; for the greatest 

 faults frequently lie under even surfaces. 



A very considerable fault is observable at Crib-bath, where the 

 beds, or strata of the limestone, stand erect. Another fault of great 

 magnitude lies between Ystradvellte and Penderryn, where all the 

 strata, and the north side of the basin, are moved many hundred 

 yards southward. 



The limestone appears at the surface, all along the boundary line, 

 in the counties of Monmouth, Glamorgan, Carmarthen, and Brecon; 

 and no doubt can be entertained that it ranges from Newton, across 

 Swansea Bay, to the Mumbles, and from Canmaddock Hill, across 

 Carmarthen Bay, to Langam Tenby. In Pembrokeshire it appears 

 at the surface only in some particular spots ; yet it certainly forms 

 an under-ground connexion from one spot to the other. 



Glamorganshire possesses by far the greatest portion of coal and 

 iron ore ; Monmouthshire is the next in point of quantity ; then Car- 

 marthenshire ; then Pembrokeshire; and lastly Brecknockshire, which 

 possesses the least. 



Observations on the Permanency of the Variation of the Compass at 

 Jamaica. In a Letter from Mr. James Robertson to the Right 

 Hon. Sir Joseph Banks, K.B. P.R.S. %c. Read June 12, 1806. 

 [Phil. Trans. 1806, p. 348.] 



The object of Mr. Robertson, who resided in Jamaica, as a King's 

 Surveyor of Land, upwards of twenty years, is to show that no 



