MR MILNE ON THE GEOLOGY OF ROXBURGHSHIRE. 473 



is clearly a chemical process, by which the red dye has been to a certain depth 

 discharged, or, in other words, the iron changed from a peroxide to a protoxide. 

 If this process could be discovered, we should in all probability have a clew to the 

 problem of the white spots. 



I may observe, that along these cracks in the red sandstone, there is often a 

 large development of metallic incrustations, having a dendritic form. The iron 

 seems as if it had been withdrawn from the general mass of the stone through 

 which it was diffused, and that thereby the stone was restored to its original 

 white or greenish- white colour. Appearances similar, or at least analogous, to 

 these, are common in the red sandstone rocks, when in contact with or near trap 

 rocks, which had risen through them. The red rocks, in such a situation, acquire 

 a brown and sometimes even a yellow colour ;* and on examining with a micro- 

 scope the structure of the stone, particles of iron are found in a state of crystal- 

 lization, instead of being equally diffused through the whole mass. 



If heat has produced these last mentioned effects, it may equally have pro- 

 duced the similar change which has taken place at the sides of cracks ; assuming 

 that these cracks were formed during the desiccation of the rocks, by the influence 

 of subterranean heat. 



In several specimens, now before the Society, of these spherical spots, 

 there is a metallic-looking pea in the centre, which would seem to indicate that 

 the iron previously diffused through the spot, had become aggregated into the 

 centre. If, as in the cases just referred to, heat was capable of making the iron 

 separate from the general mass of the stone and form metallic incrustations, might 

 it not have produced the analogous effect in the spot ?f 



* Two places, not far from one another, where these effects may be observed, are on the right bank 

 of the Tweed, one opposite to Merton House, at the Craigoer rock, and the other opposite to Dryburgh, 

 at the Holm House. 



t When this part of my Memoir was going through the press, I wrote a note to Dr MADDEJ T , stating 

 shortly the views expressed in it. From his answer I make the following extracts, as containing some 

 important suggestions : 



" I have just received your note, and, having considered its contents, would offer the following obser- 

 vations. The idea that suggested itself to me at the time of the analysis was, that the deoxidizing agent 

 producing the white spots, must, in all probability, have been a portion of organic matter in the act of 

 decomposition, this may have been a fish bone or scale, or any other organized body ; there are, however, 

 certain objections to this view of the matter, which I will now state. 



" 1st, If the spots were produced by the decomposition of any substance imbedded within its mass, 

 the effect would be produced with greatest effect in the immediate neighbourhood of the decomposing 

 body, and this effect would gradually diminish in intensity as the distance increased ; whereas, in the spot, 

 there is an abrupt transition from the deoxidized to the unaffected mineral. 



" 2d, As the intensity of effect would bo proportioned to the decomposing mass, and as the distance 

 to which the effect was produced would likewise be proportional, the exterior of the spots should possess 

 a shape either exactly or nearly similar to that of the organic body inclosed ; whereas, the spots in ques- 

 tion are, without exception, nearly spherical. 



VOL. XV. PART III. 6 M 



