36 W. M. HutcMngs — Clays, Slates, and Contact- Metamorphism. 



VI. — Notes on the Composition of Clays, Slates, etc., and on 

 Some Points in their Contaot-Metamokphism. 



By W. Maynaed Htitchings, Esq. 



ME. TEALL, in his very interesting and suggestive address 

 to the Geological Section of the British Association at 

 Nottingham, alluded to the composition of sediments (clays, etc.) as 

 compared with that of older sedimentary rocks, and specially to the 

 amount of alkali contained in them. 



At the time he made his remarks I was engaged on some analyses 

 of clays, with a view to gaining information on this particular point, 

 in connection with studies of clays and slates which I have been 

 carrying on for some years, and on which I have published papers 

 in this Magazine on former occasions. The results of these analyses, 

 and some considerations arising out of them, I propose to give as 

 the first item of these present "notes." 



For reasons which I stated on a previous occasion, I have taken 

 for special study the series of clays and shales which occur in 

 the Coal-measures, and of these I have used for detailed microscopical 

 examination only such occurrences as were not in contact with Coal- 

 seams. I did this because, wherever a bed of clay or shale has had a 

 seam of Coal immediately above it, we may assume that it has served 

 as the soil on which the vegetation grew which produced the Coal, 

 and has thus been deprived of large amounts of alkali, and so altered 

 in its chemical composition in a manner which would not apply to 

 quite similar deposits of sediment in other formations. Also beds 

 of clay and shale occurring with Coal are often broken up, — are 

 unsti'atified, — and so affected mechanically as well as chemically, and 

 rendered less suitable for the observations I had in view, 



I have always maintained, and still maintain the more strongly 

 the more I examine them, that the clays and shales of the Carbon- 

 iferous beds represent the waste of granitic or gneissic areas practically 

 " pure and simple," and that they are in all respects the counterparts 

 of the sediments which gave the materials for the principal masses 

 of slates and slaty grits of the older geological formations. But 

 there was always an apparent chemical difficulty in the way, inasmuch 

 as the published analyses of these carboniferous clays showed so little 

 alkali that they could not be supposed capable of yielding slates of 

 average composition unless some means were assumed by which 

 alkali could be added, and no such means could be suggested for 

 which there is any evidence. This difficulty was also pointed out 

 to me by others, among whom was my friend Mr. Teall himself. 



I was convinced from microscopical work, levigations, etc., that in 

 the clays examined there was much more potash-mica than would be 

 represented by the alkalies usually reported in the analyses on record. 

 In considering these analyses several points must be kept in view. 

 For one thing, just the clays which I avoided as being abnormal 

 from a geological standpoint are those of most industrial importance, 

 and therefore most analysed ; a clay being usually a better " fireclay " 

 the less alkali it contains. Then again, many of the analyses are 



