158 W. M. Hutchings — Ash-slates of the Lake- District. 



a mixture which would correspond, in round numbers, to something 

 like 50 per cent, of sericitic mica and garnets, and 50 per cent, of 

 free silica in the form of quartz with apparently some chalcedony. 

 In some slides the minute quartz-mosaic expands and gets coarser 

 at places, when its nature is more easily made out. Felspar is not 

 usually distinguishable in it. We may look upon it that the original 

 andesitic and other volcanic dust of the rock has decomposed in 

 such manner that the augite and other ferro-magnesian constituents 

 gave rise to chlorite with garnet, while the felspathic part of the 

 mixture was largely altered to mica. 



It is pointed out by Tschermak (Mineralogie, third edition, p. 467) 

 that during the alteration of potash-felspar to mica, a large pai't of 

 the silica is libei-ated, together with the greater part of the potash 

 in the form of silicate. Rosenbusch also points out the same thing, 

 and shows that soda-lime felspars undergo a similar change, which, 

 however, then involves also a separation of lime as calcite (Microscop. 

 Physiographie der Mineralien, pp. 516, 543). 



The silica thus liberated accounts for the finely diffused quartz 

 (and chalcedony) above described, while calcite, also due, doubtless, 

 in part to the same processes in the felspars, abounds in all these 

 rocks. Some of the very fine-grained bands may be seen almost 

 free from it, as is the slate we are just now considering; but it has 

 then been deposited all the more copiously in the alternating coarser 

 bands. 



Whether in these cases of mica-formation in plagioclase-felspars 

 the resulting mica is sometimes largely soda-mica (paragonite), or 

 is due only to the potash which is so usually contained in these 

 felspars, does not appear to have been ascertained. It would be 

 very difficult, if not impossible, to get decisive chemical evidence. 

 It seems reasonable to suppose that in some cases, at any rate, 

 paragonite will be formed in a manner precisely analogous to the 

 production of muscovite. So far as this Mosedale slate is concerned, 

 however, the analysis appears to show that paragonite is not present. 

 All analyses of Lake District andesites show a notable percentage of 

 potash, which there is good reason to suppose is mainly contained in 

 the felspar and glass of the ground-mass, and the ashes and tuffs 

 mainly contain also more or less of rhyolitic, and possibly trachytic, 

 fragments. 



Garnets are alluded to as present in some of the Lake rocks by 

 Mr, Ward and by Mr. Sorby, but in both cases, as far as I under- 

 stand it, in a larger form. I do not know that its presence so finely 

 disseminated and in such abundance has been pointed out. Slates 

 like these at Mosedale, full of minute garnets, occur also at many 

 other parts of the district, and it has occurred to me whether some 

 of them might not have good qualities as whet-stones conferred upon 

 them by the presence of this mineral. The celebrated " coticule " 

 of Viel Salm is considered to owe its virtues to the garnets contained 

 in it; these are, however, very much larger than the largest of the 

 grains in the Mosedale slates. 



The " base " of the slates is not all garnetiferous. It often 



