446 



MR MILNE ON THE GEOLOGY OF ROXBURGHSHIRE. 



per cent, of carbonate of lime, 44 of magnesia, 12 of peroxide of iron, and 4 of 

 silica.* 



At all the places where I have seen this chert limestone, there is contiguous 

 to, or closely adjoining it, a great abundance of trap (generally a clay stone por- 

 phyry), containing crystals of felspar as well as of augite. This porphyry, as will be 

 afterwards more fully explained, occurs in beds or strata of pretty equal thickness ; 

 and it is a remarkable fact, that the nearer the stratified rocks are to this igneous 

 rock, the more perfectly are the characteristic ingredients of the chert limestone 

 developed. Thus, at Hadden there is a stratum of chert limestone, eight feet thick, 

 interposed (with other strata) between very thick and very extensive beds of a 

 coarse amygdaloidal felspar. The superjacent bed forms along its outcrop a pre- 

 cipitous cliff, which runs along the south bank of the Tweed for nearly a mile. 

 At a distance from the trap, the solid body of limestone ceases ; but even there, 

 nodules of it exist in the beds of blue marly clay which abound in that district. 

 At Henderside Mill, as well as above Fireburn Toll (places on the north bank of 

 the Tweed below Kelso), similar beds of chert limestone exist in the near vicinity 

 of trap. 



At Bedrule Hill, there are several beds of this limestone, the position of which 

 is shewn in the following section. 



1 and 2. Porphyry beds, from 15 to 20 feet thick. 



3. Altered sandstone. 



4, 5, 6. Blue shales and slaty sandstones. 



7. Chert limestone, about 6 feet thick. 



8. Unknown, about 20 feet. 



9 and 10. Strata of chert limestone. 



11. Shales and sandstones of coal-measures. 



The foregoing section extends for about 200 yards. The greenstone porphyry 

 here, as at Sprouston and Hadden, forms beds nearly horizontal. Its outcrop at 

 Bedrule runs very uniformly above these carboniferous strata, for about a mile. 



* Analysis by Dr R. D. THOMSON. (Mag. of Nat. History, by LOUDON, No. 29.) 



