The Vitrified Cement from an ancient fort. 187 



of iron might have lent some colour to such a suggestion, 

 it is absolutely negatived by the absence of calcium and 

 magnesia, which are alv^ays present in basaltic rocks. 



For the same reason, and because of the low percentage 

 of alkalies, it cannot have been a gneissose or granitic base 

 that was used, nor (consideringthelowpercentageofalumina,) 

 can kaolin or clay have constituted any considerable portion 

 of the material. In some parts of Scotland where such forts 

 occur there are beds of red sandstone (of the old red 

 sandstone age and of formations older than this), and the 

 essential difference in composition between these lies only 

 in a rather lower percentage of iron and of alumina and a 

 little higher percentage of silica. These rocks, however, 

 approach nearer to the vitrified stone than any other 

 accessible material, and with the addition of a little iron ore, 

 or slag, might at any rate be brought in almost exact 

 agreement. The question of the source of the material, 

 in this particular case, is rendered more difficult because the 

 actual locality from which the mass was obtained is not 

 known, and in any case it would be necessary, in order to 

 solve such a question satisfactorily, to analyse samples of 

 vitrified forts from different districts, and to take the 

 results in connection with the rocks found in the district. 

 With regard to the temperature that would be required 

 to bring about the fusion of such a mass, I may call to 

 notice a series of investigations which have, during recent 

 years, been undertaken by Seger {Thonindiistrie-Zeitnng, 

 1886, p. 135,) with a view to determine the relation of fusibi- 

 lity of a mixture to the proportions in which the constituent 

 parts occur. Seger made up mixtures of silica, kaolin, 

 and marble in different proportions, until he, by means 

 of trials in a pottery furnace, arrived at a proximate 

 idea of the best proportions for obtaining low fusibility. 

 Having found this, he then made a large number of mixtures, 

 varying the several constituents, whilst keeping others in 



