172 DESCRIPTION OF THE STRATA. 



the maguesian limestone, in its progress from Cockfield to the Tees.* 

 This supposition, however, is by no means probable; for, whether 

 we consider the red sandstone of the vale of the Tees as lying over 

 the limestone, or as an tipfillmg between that and the aluminous 

 strata, it is difficult to conceive, how the dyke should traverse the 

 sandstone, without also intersecting the limestone, which is inter- 

 posed between the sandstone and the coal district. Perhaps in a 

 few spots, both in the alum hills and in the limestone, the vein may, 

 as at Whitley, be covered by a portion of the upper strata ; but the 

 thickness of the alluvial covering, in the plains which it crosses in 

 running north-west from the Tees, is sufficient to account for its 

 rarely appearing at the surface, without supposing it to be covered by 

 any part of the regular strata. At Preston quany, where the dyke 

 crosses the Tees, a little below Yarm, the alluvium is of such a thick- 

 ness, that had not the channel of the Tees made its way through it, 

 the whin dyke would not have been known to exist there. 



As this dyke intersects the strata nearly at right angles, and as 

 the dip of the strata in the alum hills is generally towards the south 

 or south-east, the dyke in traversing these hills, hades, or inclines, to 

 the same quarter. In general it runs in a straight line, N. W. and 

 S. E.; or more correctly, W. N. W. and E. S. E : but it sometimes 

 deviates from the rectilineal course. Among the alum hills we meet 

 with some deviations more apparent than real, at the crossing of the 

 valleys : for where the dyke descends from the heights, as it does 

 below Silhoue, it seems to bend toward the north ; because, as the 

 dyke leans towards the south, the higher parts of it are further south 

 than the lower. In other parts, however, there is a real deviation 

 from the straight line. For instance, the line of the dyke at Preston 

 quarry, instead of bearing in the direction of Langbargh ridge, 



* Tliis is the opinion of Mr. Winch. See Gedlog. Trans. IV. p. 35. 



