226 JF. M. Uutchings— Ash-slates of the Lair-District. 



Original labradorite. Altemiioii -product. 



Soda ... 5"43 per cent. 0-88 per cent. 



Potash... 0-92 ,, 8-51 ,, 



and in another still more extreme case the alteration-product con- 

 tained Soda = 0-17 p.c. ; Potash = 9-71 p.c. 



From this latter alteration-product acid extracted 37'61 p.c. 

 of a silicate of magnesia, alumina and iron oxides, — evidently a 

 chloritic substance. Tlie residue left after this extraction contained 

 very nearly all the potash, " and coi'responded in composition to 

 orthoclase." These alterations appear to be exactly cases of what 1 

 suppose to have taken place in some of the andesites. 



A chloritic mineral is formed, with a simultaneous more or less 

 extensive chemical alteration of the soda-lime felspar, tending to 

 convert it into orthoclase. In these Lake District rocks powerful 

 dynamic action, following on the chemical change, may be supposed 

 to have completed the work in re-crystalUzing the felspar-substance. 

 ]t will not affect the question, whether chlorite, mica or calcite is 

 formed during the chemical stages of these processes. 



In commenting on the above results of Lemberg, Roth points out 

 that it is not indicated from what source the potash was supplied 

 which was taken up during the changes of the felspars. In the case 

 of the rocks we are considering there is no difficultj'^ as to such 

 source. I have shown how frequently white mica is formed in the 

 alteration of some of the felspars of these rocks, and how specially 

 and very largely this is the case in the beds of fine ash and tuff. In 

 these ash-beds the finely-pulverized material has undergone these 

 alterations as a whole, including the portion which in the massive 

 andesites is consolidated as ground-mass, considerably richer in 

 potash than the lest of the rock, but undergoing these changes, as 

 a rule, in a very much less degree. 



These changes, as we have seen, entail a considerable liberation 

 of potash as soluble silicate, and we may be sm-e that during their 

 progress all these rocks, as a whole, were permeated by solutions 

 containing that salt, and also carbonate of potash. These solutions 

 would play an important part in further changes, and acting upon 

 soda-lime felspars (themselves containing more or less of potash) 

 already in course of decay, would easily and naturally bring about 

 the alterations supposed. No doubt similar explanations would apply 

 in the cases proved by Lemberg. 



Lemberg was apparently able to isolate the special felspars, the 

 alterations of which he wished to study. Such isolation is, un- 

 fortunately, not possible in the case of the rocks to which I refer. 

 The felspars are so small that the picking-out of them is not 

 practicable, and their isolation from the crushed rock by means of 

 dense solutions is a tedious and difficult matter. Also it is not 

 possible in this manner to separate the different varieties of felspar 

 one from the other, as even in the most favourable of the occurrences 

 examined, the included secondary minerals, though small in amount, 

 suffice to completely counteract the normal differences of specific 

 gravity. 



