338 CLEVELAND DYKE. 



into a loose, granular, saccharoid rock, in which, nevertheless, some 

 traces of organic remains, such as crinoidal column, remain. But the 

 most remarkable effect is the generation of garnets in the contiguous 

 shale under the basalt of Cronkley Scar ; a case analogous to that 

 described by Professor Henslow l in connection with the dykes of 

 Plas Newydd. 



The igneous rocks themselves are chiefly a fine-grained dark 

 basalt, changing to a coarse-grained dolerite. Contemporaneous 

 veins of very beautiful hypersthenic rock pass through the basalt 

 in several points, and it is traversed by a few productive lead 

 veins. 



The connection of several extensive basaltic dykes with this 

 great "whin sill" has been rather assumed than proved. 



These dykes pass in directions to the east-north-east, east-south- 

 east, and nearly east, and they take straight lines through all sorts 

 of rocks. Their respective breadths, and the quality of the rock in 

 each, are nearly uniform, though in these particulars they differ from 

 one another. 



Cockfield and Armathwaite Dyke. The Cleveland or Cock- 

 field dyke, in particular, ranges for seventy miles through the coal 

 series, where it chars the coal, hardens the sandstones, and whitens 

 the shales ; the lias shales and sandstones of the oolite series 

 are affected like the coal system below. Generally it is a nearly ver- 

 tical dyke, but at Cockfield Fell is subject to oblique expansions 

 of a singular kind. The dyke which passes east-north-east is remark- 

 able for having a small vein of lead ore running by the south-east 

 side of it, and for converting the shales through which it passes 

 to the state of a soft, whitish shale, called "pencil-bed," like those 

 in connection with the whin sill. It does not cut through the 

 magnesian limestone. 



This dyke, well described by Sedgwick, appears first about six miles 

 south of Whitby, near Maybecks on Swerton High Moor. It extends 

 W.JST.W. by Egton Bridge, Amthorpe, Ryehill, and Ay ton, to Mun- 

 thorpe in the Vale of Cleveland. Its thickness increases along this 

 line from 18 feet at Maybecks to 80 feet at Great Ay ton, though the 

 top of the dyke is only 20 feet thick at the latter place. In this 

 distance of 20 miles the dyke is intrusive in Oolite and Lias, and 

 forms a conspicuous feature on the moors east of Selhone. West of 

 Nunthorpe it extends by Stainton and Preston, where it makes an 

 abrupt bend. It reappears after an interval at the village of Bolam, 

 and extends to Cockfield Fell. Near Bolam it is 200 to 300 yards 

 wide; near Cockfield it terminates abruptly beneath the stratified 

 rocks, and in this district it is quite vertical. The altered shales and 

 sandstones in contact extend to a distance of 20 to 30 yards. Though 

 absent from the surface for some distance, it is proved to extend over 

 Woodland Fell to about one mile east of Middleton. 



The dyke of the Eden valley, which extends from Eenwick to 

 Armathwaite, is a portion of the same intrusive mass, and nine 



1 GeoJ. Trans. 



.ex- 



