224 TT. M. lliitchings— Ash- slates of the Lake-District. 



optic tests in convergent liglit; and in some cases, in tuffs, large 

 decayed felspar-fragments have been re-generated into mosaics of 

 clear grains in the well-known manner. 



What these felspars now are it does not seem possible to decide 

 on optic tests alone, in thin sections. The multiple twinning in 

 most cases points to plagioclase, and extinctions appear to indicate 

 albite, oligoclase, and andesine as probably the varieties. But 

 nothing short of a laborious isolation and analysis of specimens 

 could give a safe decision. It appears to be usually considered that 

 albite and andesine are the varieties chiefly produced by the dynamic 

 re-generation of decayed original felspars. 



In the paper above referred to Mr. Harker brings forward the 

 question of the formation of new felspar by deposit in vesicles, and 

 describes a case of such an occurrence in one of the Cross Fell 

 rocks, the felspar in question being well-twinned plagioclase with 

 extinction-angles pointing to albite or andesine. 



According to my own observations, this mode of occuri-ence is not 

 by any means very rare, and I can point to several cases of cavities 

 in these rocks which are lined with chlorite, etc., and contain felspar 

 in such a manner as to apparently forbid any other explanation than 

 that the mineral has been crystallized from infiltrations into the 

 cavities. The question first attracted my attention in connexion 

 with the altered rocks round the Shap granite. Messrs. Harker 

 and Marr allude to felspars formed in vesicles by the metamorphism 

 of their contents. They refer to one particular slide in which this 

 is observed, but state that it does not seem to have commonly 

 occurred. Whether it be due to contact-action or not, my own 

 impression is that it is very usual at some points round the granite, 

 as I have several slides in which well-defined felspar, often in good 

 large individuals, frequently occurs in A^esicles with quartz, biotite, 

 hornblende, etc. It is mostly plagioclase, but I have one slide, 

 which I have submitted to Mr. Harker, in which occur grains of 

 what seems to be orthoclase, perfectly clear, beautifully cleaved, and 

 extinguishing quite parallel to the cleavages. One such grain is 

 over ^ inch in diameter filling an irregular cavity, which may have 

 been a vesicle, but seems more likely to have been due to the 

 removal of some former mineral. I at first attributed these large 

 grains, as also the smaller plagioclase in vesicles, to the action 

 of the granite, and this may be correct, but it is not necessarily 

 so, as later observations showed me that all these occuirences are 

 paralleled in rocks far outside the contact-zone. Cavities with 

 plagioclase, and apparently also orthoclase, occur not rarely in the 

 coarser slates at Motsedale, for instance; and tiie grains of well- 

 cleaved felspar, apparently orthoclase, are seen again in sections 

 of the andesite of Barter Fell, Mardale, where the minei'al occurs 

 in a precisely similar manner as large, irregulai", perfectly glassy- 

 clear, untwinned grains in a rock whose original fel^^pars are all 

 sharply and definitely bounded, and are now exceedingly turbid. 



I look upon all these cases as due to causes wholly distinct from 

 the dynamic regeneration of decayed felspars, and as explained by 



