BASALTIC DYKE. 173 



diverges to the south about twenty degrees, and would, if prolonged, 

 arrive at Battersby, Ingleby, or Greenhoue> 



It is observable, that this large vein is thickest in the middle, 

 and thinnest at the two extremities. At Cockfield, its width is but 

 17 feet ; at the quarries of Preston and Langbargh, it is 70 feet, or 

 more; at Silhoue, it is diminished to about 30 feet ; and at Maybecks, 

 its eastern termination, it seems to be scarcely any wider than at Cock- 

 field. Thus the vein appears to swell in the middle, and to run to a 

 narrow edge at the extremities. As such dykes, however, are justly 

 regarded as large fissures or breaks in the strata, filled with basalt, 

 the thickness of the dyke at the bed of the Tees and in the plain of 

 Cleveland, may be otherwise accounted for'; because, on the suppo- 

 sition that the vein has been filled from below, we may expect to find 

 it widening as we descend ; and consequently, it must be wider in the 

 vale of the Tees, than in such elevated situations as Silhoue and 

 Cockfield Fell. Yet our observations do not warrant us, to attribute 

 the difference of width in this vein, solely or principally to a differ- 

 ence of depth ; for it appears of a great thickness at Cliflrigg, where 

 it is protruded to a considerable height, as well as at Langbargh 

 quarry, which lies in the plain immediately below. We may add, at 

 the same time, that wherever the ridge protrudes greatly above the 

 surface, it occupies a much wider space than the mere thickness of 

 the vein; for the higher portions of the ridge, having nothing to sup- 

 port them, have fallen down on both sides, especially on the south 

 side to which the dyke leans. 



The basalt in this dyke is disposed in blocks of various sizes and 

 forms, generally oblong, and lying across the vein, parallel to one 

 another, and sometimes approaching the form of columns, but with 

 little regularity. One of the nearest approximations to the columnar 

 form may be seen at Cliffrigg, where the blocks are often long, and 

 appear like rude pillars, lying piled up horizontally. The blocks are 



2 w 



