226 James JYeilson — Old Red and Carboniferous of Arran. 



and gentle baking, to which they must have been subjected, is duo 

 the alteration to red oxide, both in the ironstones and sandstones. 

 That bricks assume their red colour by firing is common know- 

 ledge ; and, although a heat sufficient to burn a brick might destroy 

 a fossil in the limestone or shale, yet the heat required to effect the 

 change in colour may not be so great after all — as one sees in the 

 case of the lobster that is affected by the heat of boiling water. 



I throw out this suggestion with considerable diffidence, knowing 

 it to be open to many and serious objections ; but it is the only 

 hypothesis I can suggest, and even should it prove to be erroneous, 

 it may evoke a better and truer solution. 



Since the above remarks were penned a curious circumstance has 

 come under my notice, which has a bearing on this point. Some 

 time ago, during excavations for a gas-holder tank at Thornhibank 

 works, the excavated material was carted away and thrown down at 

 Arden old quarry. It proved to be a fossiliferous shale, containing 

 a large number of h'onstone nodules. After lying some time it took 

 fire, owing probably to spontaneous combustion, and has been 

 smouldering away for the last month or two. The heat in the 

 deeper parts was probably great, but on the surface was not 

 very high — perhaps from 120° to 150° Fahr. ; yet this heat has been 

 sufficient to alter the colour of the ironstone nodules from dark- 

 brown or blackish to red, as shown in specimens obtained by me, 

 some of which contain fossils which have been altered but not 

 destroyed. 



Another point is the condition of the fireclay and shales charged 

 with plant-remains. The presence of the carbonized plant-remains 

 turns the shales and fireclays in which they occur, black, and 

 I believe this to be the invariable rule, and feel pretty safe in 

 saying that any which are not black have been subjected to alteration. 



The effect of heat is to drive off the carbon and leave the shale 

 white, or, if it contains iron, reddish — a fact familiar to every house- 

 wife in the country who buys stones along with her coals. Then 

 there is the fact patent to all, that bricks, whether made from fireclay 

 or dark carbonaceous shales, are white. 



Now, none of the shales or fireclays among the red rocks of 

 Arran are black ; they are either of a creamy white or reddish colour. 

 That alteration, as we have shown, could have been produced (and 

 I am inclined to think has been produced) by heat. 



Proceeding from observation to experiment, I took some specimens 

 of fireclay filled with small ironstone nodules of a dirty black colour 

 (which had been discovered by Mr. J. B. Wyse at Blochaim, north- 

 east of Glasgow) and put a piece into the parlour fire, heaping 

 coals over it. After about two hours roasting it was found that the 

 black colour was gone, the matrix was changed to a white colour, 

 and the ironstone nodules had been altered to a blue colour. As, 

 however, I was under the impression that too much heat had been 

 applied, another specimen was put into the fire before retiring for 

 the night, and in the morning the result was more satisfactory. 

 The matrix was white, and the iron in the condition of red oxide. 



