228 J, J. H. TEALL—-PETROLOGICAL NOTES ON 
Tue Hert Dyke. 
Literature. 
Sepewick, Rev. Apam. ‘ Geological Structure and internal Rela- 
tions of the Magnesian Limestone,” Trans. Geol. Soc. 2nd ser. 
vol. 111. part i. 
‘*On the Phenomena connected with some Trap-dykes in 
Yorkshire and Durham,” Trans. Cambr. Phil. Soe. vol. ii. 
Bs, I. Lowrutan. “On some supposed Changes Basaltic Veins 
have suffered during their passage through and contact with 
Stratified Rocks, and on the manner in which these rocks have 
been affected by the heated Basalt,” Proceedings of the Royal 
Society, 1875, p. 543. 
This dyke crosses the following 1-inch Ordnance Maps: 108° N.E., 
N.W., and 8.W. Its course is thus traced by Professor Sedgwick * 
on the authority of Mr. Wharton, of Oswald House, Durham :—“ It 
ranges from the escarpment of Magnesian Limestone (at Quarrington 
Hill, a few miles to the east of Durham), through the great coal- 
field, in a direction about W.S.W. It is found along this line at 
Crowtrees, Tarsdale, Hett, Tudhoe, Whitworth, and Constantine 
Farm. From the last-mentioned place it passes along the same line 
of bearing through the collieries of Bitchburn and Hargill Hill to a 
spot near the confluence of the Bedburn Beck and the river Wear, 
where it is well exposed at the surface of the ground; and it is 
known to pass up the Bedburn-Beck valley to Kgglestone Moor.” 
The author also states that the dyke increases in width in its pro- 
eress westward from 64 feet at Crowtrees to 15 feet at Bitchburn 
Beck. 
The dyke has formerly been quarried on an extensive scale be- 
tween the villages of Tudhoe and Hett. At present (1882) it is 
worked by means of shafts at Tudhoe, near the Darlington road, 
and at Hett village. Between the Darlington-road shaft and Hett 
village its course is marked by a nearly continuous trench, the 
result of old quarrying operations, which terminates at the village 
in a vertical face of rock. The direction of this trench is nearly 
east and west. At Hett the dyke is 10 feet wide, and hades to the 
north at a very high angle. The adjacent rocks are coal-shales 
which have been baked and indurated by the action of the igneous 
mass. A rude spheroidal structure has been brought to light by 
the action of the weather. This dyke, like the one already de- 
scribed, appears to keep its character for great distances; and 
although I have only examined it between Tudhoe and Hett, I have 
no doubt that the following petrological description will apply to it 
throughout its entire length. 
Macroscopically it is a fine- or medium-grained crystalline rock of 
a dark grey or bluish-grey colour and a subconchoidal fracture. 
It can be at once distinguished from the rock of the Cleveland dyke 
by the absence of any porphyritic crystals of felspar. Here and 
* Trans. Cambr. Phil. Soc. vol. ii. (1827). 
