MR MILNE ON THE GEOLOGY OF ROXBURGHSHIRE. 475 



by which they are characterised in other parts of Scotland. 1. In Fife and Moray- 

 shire the formation is described as consisting of yellow, grey, and dark-red beds, 

 the first of these being the highest, and the last the lowest in the series. It has 

 been seen that these varieties, and in the same order, characterize also the old 

 red sandstone formation of Roxburghshire. 2. In Fife this formation is overlaid 

 by the coal-measures, just as in Roxburghshire. 3. The remains of the Holopty- 

 chius, and of a smaller fish much akin to it, which characterize the old red sand- 

 stones in the North of Scotland, have been found among the red rocks of Rox- 

 burghshire, in several localities. It is also not a little remarkable, that these fos- 

 sils should be found throughout Scotland, characterizing only the red and yellow 

 beds,* but not the intermediate grey beds. 



(3.) It has been assumed in the remarks above made, that the strata of the 

 coal-measures in this district have been derived from the finer debris and sedi- 

 ment afforded by the grey wacke and felspathic rocks. It is well known that these 

 ancient rocks contain, in general, all the elements which are necessary to form 

 beds of sandstone, shale, limestone, and magnesia. -j- The porphyritic rocks of the 

 Cheviot do in many places contain lime (at least they effervesce with acids^), as 

 also great abundance of silica and alumina. At the same tune it is difficult to 

 perceive how these different substances, brought simultaneously, and forming a 

 common sediment, could have been deposited in separate beds of pretty uniform 

 thickness, and of great extent. It seems more natural to suppose, that all the 

 particles of these different substances, mechanically suspended, were deposited 

 promiscuously in one common mass ; and that some movement of the particles 

 afterwards took place, probably according to their respective chemical affinities. 

 Some of the elements which now occur in the strata, of course, were held by 

 the water in solution, as, for instance, carbonate of lime and carbonate of mag- 

 nesia ; and it is not difficult to conceive how these may have been precipitated 

 according to circumstances. 



Thus, the extensive beds of chert limestone, the thin beds of magnesian lime- 

 stone, and the nodules and veins of gypsum or sulphate of lime, which occur (as 

 was shewn in the first part of this Memoir) only near great sheets of porphyritic 

 trap, probably owe their origin to the great and long-continued heat in those places 

 where they occur. It is well known to geologists, that the frequency with which 

 gypsum and magnesian limestone or dolomite are associated, has long been mat- 

 ter of speculation, a circumstance which, as Sir HENRY DE LA BECHE observes, 



* For an account of the Fife fossils, see the Rev. Mr ANDERSON'S Memoir, published in the High- 

 land Society's Transactions ; and of the Moray beds, see Sketches by PATRICK DUFF, Esq. 



f DE LA BECHE, Manual, p. 450. 



{ The porphyry at Plewlands effervesces very briskly, and must contain a large quantity of lime. 



