476 MR MILNE ON THE GEOLOGY OF ROXBURGHSHIRE. 



" has not been satisfactorily accounted for." * In the work now quoted from, 

 Sir HENRY refersf to the opinion of VON BUCH, that the dolomite of the Alps and 

 some other places is an altered rock, and has been acted on by the augite por- 

 phyries, Avhich contain magnesia, and from which, therefore, the magnesia may 

 have been derived. If the magnesia has been derived only from the porphyry, 

 it is not easy to understand the transmission of it to beds at a distance from the 

 porphyry. It seems to me more natural to suppose, that the water diffused 

 through the sedimentary deposits held magnesia, as well as other substances, in 

 solution ; and that, by an excess or long continuance of heat, a precipitate was 

 caused, which would be diffused through the beds, and act as a cement to the 

 particles of silex and alumina, and other substances which had been mecha- 

 nically deposited. The same remark I would apply to the beds of chert at Had- 

 den, Bedrule, and other places, containing nodules of chalcedony and of lime, 

 which can scarcly be doubted to have been chemical precipitates. In like man- 

 ner, the nodules of red and the veins of white gypsum which occur in the marl 

 strata, may be easily supposed to have been thus formed. 



The abundance of lime, in one form or other, existing in the sedimentary 

 rocks of the district, is very extraordinary ; and appears to be due to some other 

 cause, than the mere wearing down of the Cheviot porphyries. For though, as al- 

 ready mentioned, these porphyries occasionally effervesce with acids, the quantity 

 of lime thus indicated, bears no proportion to the quantity existing in the old red 

 sandstone and carboniferous formations. There are few places, where the sand- 

 stones of both sets of rocks do not effervesce. The red sandstone at Lochton, 

 about two miles east of Kelso, yielded on analysis 25 per cent, of lime.:): The well- 

 water at Eccles Manse, in Berwickshire, shewed, out of 100 parts, 57.75 of sul- 

 phate of lime, and 29.75 of common salt. It seems to me, therefore, that the 

 waters in which these sedimentary rocks were deposited, and which continued to 

 saturate the sediment out of which they were formed, must have contained in 

 chemical solution a large proportion of lime. 



Allusion was made in the first part of this Memoir to the existence of "yokes 1 '' 

 or large concretions, not merely in the igneous but also in the sedimentary rocks, 

 and especially the sandstones. The formation of these harder portions seems due 

 to chemical action of some sort, excited probably by heat. 



I am disposed to ascribe to the same cause the formation, at least in many 

 instances, of beds of homogeneous matter. It is difficult to imagine how strata, 

 which sometimes extend uninterruptedly over large tracts of country, and pos- 

 sessing a remarkable uniformity of thickness, could have been formed by the mere 



* Manual, p. 478. t Ib. p. 475. 



\ Analysis by Dr THOMSON, given in London's Mag. of Nat. History. 



