54 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



operations, at a height of 400 feet above sea-level, and the dyke has there been ascer- 

 tained to be 80 feet broad. Higher on the hill, close to the 750 feet contour-line, its 

 breadth is only 20 feet, so that it narrows upward as much as 60 feet in a vertical height 

 of 350 feet. Its contraction in width during the last twenty feet is still more rapid, and 

 in the last few yards it diminishes to two or three feet, and has a rounded top over which 

 the strata are bent upward. The accompanying section (fig. 8) across the upper part of 

 the dyke will make these features clearer. 



Further to the west an exposure of the upper limit of the dyke has been described 

 and figured by Mr Teall. In 1882, at one of the Cockfield quarries (fig. 9), the dyke 





Fig. 8. — Section across the extreme upper limit of the Cleveland 

 Dyke, Ayton, on the scale of 20 feet to one inch (G. Barrow). 

 a, Jurassic shales, &c. ; b, Dyke. 



a b 



Fig. 9. — Upper limit of the Cleveland Dyke in 

 quarry near Cockfield (after J. J. H. Teall). 

 a, a, Carboniferous shales ; b, Dyke. 



was "seen to terminate upwards very abruptly in the form of a low and somewhat 

 irregular dome, over which the Coal Measure shales passed without any fracture, and only 

 with a slight upward arching." * 



Near the other or north-western termination of this great dyke, similar evidence is 

 found of an uneven upper limit. After an interrupted course through the Alston moors, 

 the dyke reaches the ground that slopes eastward from the edge of the Cross Fell 

 escarpment. Its highest visible outcrop is at a height of 1 700 feet. But westwards from 

 that point the dyke disappears under the Carboniferous rocks, and does not emerge along 

 the front of the great escarpment that descends upon the valley of the Eden, where among 

 the naked scarps of rock it would unquestionably be visible if it reached the surface. 

 Its upper edge must rapidly descend somewhere behind the face of the escarpment, 

 for the igneous rock crops out a little to the west of the foot of the cliff, at a height of 

 about 1000 feet below the point where it is last seen on the hills above. Here the top 

 of the dyke has a vertical drop of not less than 1000 feet, in a horizontal distance of 

 five miles, as shown in fig. 10, which has been drawn for me by Mr J. G. Goodchild. 



* Quart. Jour. Geol. Soc, xl. p. 210. 



