Northumberland and Durham. 25 



Cockfield to Botain, situated on the western boundary of the mag- 

 nesian limestone. Its width is 17 feet at the former place, where 

 it hades to the south, and throws up the coal-measures on that side 

 3 fathoms. The low main coal contiguous to the basalt is only 9 

 inches thick, but enlarges to 6 feet at the distance of 50 yards from 

 it. The coal is reduced to a cinder, and the sulphur is sublimed 

 from the pyrites near to the dyke. 



I have never been able to trace any of these basaltic veins into 

 the magnesian limestone, and am almost certain that together with 

 the other members of the coal formation, they are covered by it. 



Continuing the line of direction of the Cockfield dyke from 

 Botain to the south-east, after passing the eastern boundary of the 

 magnesian limestone, we meet with a dyke on the banks of the 

 Tees a little below Yarm. It there cuts the red sandstone, and 

 continuing its course in the same direction is well known to tra- 

 verse the north-eastern part of Yorkshire. 



Besides the fissures filled with basalt, others of a very different 

 nature intersect the Coal-field. These if large are also called dykes , 

 but, if inconsiderable, (roubles, slips or hitches j and are the same 

 that some geologists have called faults, 



I have already noticed the main or ninety-fathom-dyke \ when 

 speaking of the limestone quarry at Whitley, where it is seen 

 dividing the coal-measures in the cliff, and passing into the sea. 

 It receives its name from the degree of throw which generally 

 attends it in the strata through which it passes, and which are 

 cast down on the northern side about 90 fathoms. At Whitley the 

 same bed of coal which is found at 7 fathoms on the southern side 

 of it, is found at 50 on the northern, the measures being there 

 thrown up on the southern side 43 fathoms. From this point the 

 dyke ranges, though not in a straight line, through that part of 



Vol. iv. D 



