MR MILNE ON THE GEOLOGY OF ROXBURGHSHIRE. 447 



At Roberts Linn, the following section illustrates the position of the chert 

 limestone. 



A A, Bed of porphyry. 



B B, Stratum of red clay, streaked with thin white lines, parallel to the porphyry. 



C C, Stratum of red clay, streaked with thick blue lines, parallel to do. 



D D, Bed of chert, pretty compact. 



E E, Bed of chert, mixed with blue clay. 



F F, Bed of gravelly red clay. 



G G, Ditto. 



H H, Channel of burn. 



This section is about 30 feet high, and about 100 feet long. 



Below Winshielknow, the trap in contact with the chert limestone is not (as 

 in the cases just mentioned) a plateau or horizontal flow of trap, but a dyke 

 30 feet wide which cuts vertically across the strata, running in a northerly direc- 

 tion. It has tilted the strata on each side of it. 



(3.) The only place where I noticed beds of true Magnesian Limestone, is in 

 Tweeden Burn, on the south side of Liddesdale. There were several strata, a few 

 inches thick, of a buff yellow colour, and presenting cavities in the way usual in 

 this kind of limestone. There are thin beds, precisely similar, on the banks of 

 the Tweed, both above and below Coldstream. 



Another rock (if rock it can be called) belonging to this class, which here 

 requires notice, is Gypsum. It exists in considerable abundance on the north 

 bank of the Tweed at Kelso ; also about a mile above Floors Castle, and near 

 Birgham, on the confines of Berwickshire. It has been found too on the north 

 bank of the Eden, about a mile above its junction with the Tweed. Both the 

 white and the red varieties are found at all these places. The red exists in no- 

 dules or concretions ; the white is in the form of veins, more or less vertical, 

 and has evidently been formed subsequently to the red, for it intersects the red 

 gypsum. This mineral occurs only in beds of blue marly clay. Another place 

 where gypsum is said to have been found is Archerbeck Burn, in Liddesdale. If 

 this be true, it is probably in the new red sandstone formation, which will now be 

 noticed. 



