40 W. M. Hutchings — Clays, Slates, and Contad-MetamorpMsm. 



in granites, and also in andesites and other rocks, confirms me in 

 the opinion that there also, though both occur, micaceous decom- 

 position is very much more usual than kaolinitic. And it is quite 

 reasonable to suppose that when kaoline is deposited together with 

 much felspar, biotite, muscovite, etc., the changes which afterwards 

 take place, involving as they do a micaceous alteration of the felspar 

 with liberation of alkaline silicate in solution, may effect a conversion 

 of more or less of the kaoline into mica, a process chemically quite 

 simple, and quite probable under the conditions of pressure and 

 temperature we know to have existed. 



The examination, microscopical and chemical, of the materials 

 from this bore-hole has also been of interest as showing the very 

 nnifoi'ra conditions as to nature of deposit of this not inconsiderable 

 thickness. It will be seen that the five seams of fine-grained clay, 

 separated though they are by very much coarser beds, and deposited 

 at what must have been good long intervals of time, yet show only 

 comparatively small variations in composition. 



On a future occasion I hope to deal with some of the more modern 

 clays, having difi'erent compositions from those now under consider- 

 ation, and I think also more or less different origins. Clays such as 

 I allude to have not, I consider, played any noticeable part in 

 providing the materials for the older slates and grits. 



In the Geological Magazine for October, 1891, I contributed 

 some " Notes on the altered Coniston Flags at Shap," in which, 

 among other things, I dealt with the question of newly-formed 

 felspar in these rocks and with the nature of the " clear spots " 

 observed in them. To these two points I wish now to recur, having 

 continued to study these special rocks at Shap, and also having paid 

 much attention to similar rocks in general. 



As regards newly-formed felspar at Shap, Harker and Marr, in 

 their work on these rocks, had stated that they looked on its presence 

 as probable, but they did not definitely assert it. I had made every 

 effort to make sure on this point, but was obliged to state that I 

 could not obtain any proof, by microscopic tests, of its presence. 



I must now retract this and say that not only is new felspar 

 present in these rocks, but that it is at some parts of the exposure 

 very abundant in the " mosaic " which is well developed near the 

 granite. This altered conclusion is due to some extent to the 

 additional number of very thin sections examined, of which I have 

 now a series from over twenty points along the limited exposure in 

 Wasdale Beck ; but is due much more to improvements in the 

 appliances I have used, A few remarks on this subject may not be 

 out of place here. 



Everybody who has tried to do much work on minute mineral 

 sections in convergent polarized light, using, say, | inch objectives, 

 knows what a terribly unsatisfactory thing even the best rotating 

 stage is. It is not too much to say that " that way madness lies." 

 I had a " Dick " microscope made for me by Swift and Son, fitted 

 with a good condenser. The benefit of the change to this instru- 

 ment can only be fully understood by those who have tried it ; one 



