Br. A. Hubert Cox—South Staffordshire Fire-clays. 6S 



derived from higlily aluminous shales, have been converted into 

 sillimanite-hornfels, that is, rocks consisting "entirely of minute 

 slender needles of sillimanite set in a colourless glassy base ".^ One 

 such rock has been analysed. 



It is noteworthy that these vitrified rocks occur, as a rule, in 

 association with those basic intrusive rocks, such as olivine-dolerites, 

 camptonites, mouchiquites, etc., that show a tendency to contain 

 more or less analcite. The magmas that give rise to such rocks, 

 therefore, must have been richer in water-vapour than is normally 

 the case in igneous magmas. Accordingly it may well be that the 

 vitrifying action which they exerted on the surrounding sediments 

 was facilitated by the presence of the hot vapours. This conclusion 

 is supported by the analyses quoted by Dr. Flett,- which show an 

 accession of water to the vitrified rock as compared with the 

 unaltered rock. 



It may well be that the vitrification of the ignited fire-clays was 

 likewise facilitated by the presence of slight traces of water that had 

 escaped volatilization during the preliminary heating to the biscuit 

 stage. In this connexion it may be noticed that most glassy rocks, 

 such as the pitchstones and tachylites, show a high water-content. 

 Obsidians, however, form an exception to this rule. 



Summarizing the results we may say that the artificiallj- heated 

 clay, originally cryptocrj'stalline in texture, shows first a vitrifica- 

 tion, followed by a partial devitrification, resulting in the formation 

 of the high-temperature mineral sillimanite. The texture still 

 remains very fine-grained. 



The naturally heated rocks, on the other hand, do not normally 

 show signs of any vitrification having taken place. Eather do they 

 pass into rocks of the holocrystalline hornfels type with a medium 

 to coarse texture, and in which low-temperature minerals may be 

 found either alongside, or to the complete exclusion of, high- 

 temperature minerals. Such contact-action was therefore brought 

 about by a comparatively small rise of temperature, extending, 

 however, over a considerable time. 



Exceptionally there do occur vitrified rocks comparing more 

 closely with the artificially altered fire-clays, but such vitrified 

 rocks are only associated with special types of igneous rocks, and 

 water-vapour probably played an important role in determining their 

 special features. 



I have to express my indebtedness to Professor Sir Herbert 

 Jackson, K.B.E., F.R.S., for his most valuable advice and for 

 reading through this paper, and to the Controller of the Optical 

 Munitions and Glassware Department of the Ministry of Munitions 

 of War for permission to publish these results of an investigation 

 primarily undertaken on behalf of that De{)artnient, and carried out 

 in connexion with Sir Herbert Jackson's researches on fire-clays for 

 the Clay Research Committee of the Institute of Chemistry. 



^ A. E. Radley, Summary of Progress for 1914 (Mem. Geol. Surv.), 1915, 

 p. 57. 



^ Op. supra cit., p. 132. 



