38 W. M. Sutchings — Clay, Slates, and Contact-MetamorpMsm- 



guide, we may reckon its amount as rather over 1 per cent. Its 

 determination was not important for the purpose of these analyses. 

 The iron present is all given as ferric oxide, though a small part of 

 it is in the state of ferrous oxide. This partly causes the excess in 

 the analyses. Traces of phosphoric and sulphuric acid, etc., have 

 not been determined. From these analyses it will be seen that these 

 clays would be capable, chemically considered, of transformation 



into very typical " clay-slates." Mineral ogically they are clay-slates, 

 having already undergone all, or nearly all, the mineral changes 

 requisite to constitute the normal (unaltered) slates. Nothing more 

 is needed but physical changes, such as compacting, arrangement of 

 mica in a plane, increase of size of mica, etc. 



The analyses of slates are very numerous, and there is no need to 

 burden this paper with a reproduction of them ; but, for comparison, 

 I have taken, at random, eight analyses given by Both in his 

 ''■ Allgemeine Geologie," 1890, among a list of analyses of slates 

 from various parts of Europe. 



Maximum 

 Minimum 



Average 



Silica. 

 65-84 

 60-01 

 57-93 



Alumina. 

 27-93 

 17-10 

 22-76 



Potash and Soda. 

 6-06 

 3-26 

 5-07 



Or again, taking nine analyses given in Eenard's work on the slates 

 of the Ardennes, the figures are — 



Maximum 

 Minimum 

 Average 



Silica. 

 61-57 

 45-60 

 57-20 



Alumina. 

 31-95 

 15-96 

 21-87 



Potash and Soda. 

 6-07 

 2-39 

 4-36 



It will be seen how striking is the approximation of the average 

 figures of these seventeen analyses, and also how closely they agree 

 with the average figures in the analyses of fireclays made by me. 



Further, in making these analyses I had in view the question as 

 to whether or not kaoline is present in these clays to any extent. 



I have not been able to definitely satisfy mj'self by microscopic 

 examination on this point, but have rather come to the conclusion 

 that it is either absent or only very subordinate, and I think the 

 analyses confirm this view, as regards these special samples at all 

 events. 



Seeing that no felspar is detected in them, we may assume that the 



