62 Br. A. Hubert Cox — South Stafordshire Fire-clays. 



free silica in the form of a quartz-flour, the Si O2 : AI2 O3 ratio still 

 remained in excess of that demanded by sillimanite. This excess 

 of silica could not be readily removed by any simple process of 

 elutriation in consequence of the extremely fine-grained character of 

 the quartz-dust, the particles of which become comparable in size 

 with those of the clay-base itself.^ In order, therefore, to brinj;; the 

 Si O2 : AI2 Os ratio down to that demanded by sillimanite, alumina in 

 various forms was added to the clays in various amounts. The 

 refractoriness of the clays was then again found to be further 

 increased, or in other words the softening-point was very consider- 

 ably raised. 



Turning now to a comparison of the changes occurring in 

 artificially heated clays with those in contact-altered argillaceous 

 rocks, there are certain points of resemblance, as is only to be 

 expected. But there are also, in most cases, some very interesting 

 differences in behaviour. 



In the first place it will be noted that the aluminium-silicate 

 formed under artificial heating was the high-temperature form 

 sillitnanite. In no case was the low-temperature form andalusite 

 observed, and, so far as I am aware, the artificial production of that 

 mineral has never been recorded. 



In contact-altered rocks, on the other hand, both minerals may 

 occiir, sometimes exclusively the one or the other, sometimes both 

 together, whereas in yet other cases hjanite is the characteristic 

 mineral. 



Again, the more obviously contact-altered rocks are almost always 

 holocrystalline, belonging to, or approximating to, the granular 

 rocks classed as liornfeh. Cases of vitreous rocks produced by 

 contact-action are the exception rather than the rule. We have, 

 however, examples of such vitreous rocks in the buchites, or vitrified 

 phyllites, British specimens of which have been described from 

 Argyllshire by Dr. Flett.^ 



In these vitrified phyllites, however, the newly-formed aluminium 

 silicate occurs, not in the pure state as andalusite or sillimanite, but 

 combined with Mg as cordierite. This, however, is merely the 

 result of the high magnesia-content of the original phyllite, in which 

 the magnesia was present in the form of chlorite. 



Apart from the buchites certain other examples of contact- 

 altered rocks are known that match in all respects the products 

 formed by the continued ignition of fire-clays. They are rocks that 

 occur as xenoliths in basic intrusive rocks in Mull. A complete 

 account of the phenomena there shown is not at present available, 

 but some mention of the rocks has been made by the ofiicers of the 

 Geological Survey ini-ecent Summaries of Progress. These xenoliths, 



^ It has been shown possible to remove this silica-dust by osmosis, a process 

 that has been claimed to yield excellent results in other cases [W. E. Ormandy, 

 Trans. Eng. Cer. See, vol. xii, p. 36, 1912-13, and vol. xiii, p. 35, 1913-14]. 

 When a suspension of clay is electrolized the quartz remains neutral, the clay- 

 substance goes to the - pole, while most of the impurities go to the + pole. 



" Geology of the Country near Oban and Dalmally (Mem. Geol. Surv.), 1908, 

 p. 129, with references. 



