474 MR MILNE ON THE GEOLOGY OF ROXBURGHSHIRE. 



(2.) Allusion has just been made to the coal measures, as being separated 

 from the greywacke formation by the old red sandstone group of rocks. I have 

 explained, in the former part of this Memoir, that I do not consider the coal- 

 measures of this district as forming, with reference to the old red sandstone, a 

 separate and independent formation. On the contrary, it appears to me, for the 

 reasons already given, that the two merge into each other by insensible grada- 

 tions.* It is evident indeed, that, supposing the materials of both groups of rocks 

 to have been derived from the same quarter, they would arrange themselves pre- 

 cisely as they are found to exist, viz., first the sediment loaded with peroxide of 

 iron, and afterwards the finely comminuted clays which afterwards constituted 

 the shales and limestone. 



In regard to the old red sandstone rocks of this district, I may farther observe, 

 that they appear to present the same general characters, and even the very varieties, 



" 3d, The fact of the spots being annularly stratified, the rings being in most cases distinct, and 

 easily separable by cleavage, militates somewhat against the hypothesis of the creating cause being placed 

 in the centre, because it is generally found that annularly stratified masses grow by deposition upon a 

 central nucleus ; whereas, when a central substance influences a surrounding mass, previously deposited, 

 a section generally exhibits radiations in place of rings. 



" With regard to your other suggestion, it is exceedingly probable that the iron was brought in con- 

 tact with the calcareous sand, in the form of a solution of protoxide, and that the protoxidation was an 

 ulterior effect, possibly of heat. I do not, however, see exactly what state of things could exist so as to 

 prevent the protoxidizing of particular spots, and, at the same time, to change so materially their structure. 

 Some very interesting experiments have suggested themselves to me, by which I fancy we could arrive at 

 a somewhat satisfactory conclusion respecting 1 their origin and formation. I cannot, however, as yet pro- 

 mise to undertake these experiments." 



I had suggested, in my note to Dr MADDEN, whether clay or sand, deposited in water which held 

 protoxide of iron in solution, would not, on exposure to heat, acquire a red colour, like common bricks or 

 house-tyles when put into a kiln ? The only difficulty is, to explain how, in particular spots, the per- 

 oxidation of the iron was prevented or subsequently neutralized. But if organic matter of any kind, (such 

 as fish-bones or scales), containing phosphoric or carbonic acid, existed in these spots, then their organic 

 matter would become gradually decomposed, and the acid being set free, would combine with a portion of 

 the iron to form a protoxide, and thus discharge the red colour. 



So also, in regard to the cracks and fissures, on each side of which there is a ribbon of a white or 

 greenish-white colour, may the peroxide originally, in that part of the stone, not have combined with the 

 carbonic acid of the air and water, permeating these cracks, and produced similar effects ? 



* LORD GKEENOCK, to whom, as a member of Council, this Memoir was referred for examination, has, 

 in reference to this point, written on the manuscript the following remarks : " According to Miller, who 

 quotes the opinion of Agassiz, the remains of Holoptychius are characteristic of the upper beds of the 

 old red sandstone, the inferior beds being distinguished by different organic fossils, viz., the midstone or 

 cornstone formation, by the Cephalaspis, and the lower by Ptericthys, Coccosteus, Diplopterus, &c., 

 each formation having its distinct group. Therefore, the remains of Holoptychius only having been 

 as yet noticed in Roxburghshire, is a strong confirmation of Mr Milne's views in respect to there being 

 little, if any, difference in age between the two descriptions of sandstone which he has noticed as existing 

 in that county ; scales, &c. of Holoptychius being likewise met with in the coal formation." G. 



