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



slates contain it in vaiying amount, sometimes very scarce and 

 sometimes in crystals so small as to require very careful search for 

 its detection, but at other times very abundanth' and of larger sizes, 

 ranging up to 30-0 oth of an inch in length. 



The various experiments on record as to the artificial formation 

 of crj'stallized titanic acid by sublimation show that variations of 

 temperature cause variations in the resultant crystal-form ; — rutile, 

 anatase or brookite being obtained at different parts of the apparatus. 

 Similarly the three minerals may be obtained by fusion of titanic 

 acid in different solvents. The same no doubt applies as regards 

 solutions of titanic acid in liquids, and its crystallization out of them 

 under various conditions ; but there appear to be very few experi- 

 mental data as to this, and we can only speculate as to the exact 

 nature of the solutions which have acted in these rocks. It is stated 

 by Doelter (Allgemeine Chemische Mineralogie, p. 155) that by 

 means of water containing sodium fluoride he succeeded in re- 

 crystallizing titanic acid in the form of rutile. He also gives 

 interesting figures [loo. cit. p. 189) showing a very noticeable 

 solubility of titanic acid (powdered rutile) even in pure water, in 

 sealed tubes atSO°C. From various considerations it is probable 

 that the solutions acting in these rocks were largely charged with 

 alkaline silicate ; a solution of alkaline titanate was probably formed 

 at the same time, out of which carbonic acid would liberate titanic 

 acid. It seems likely that so far as concerns temperature, pressure, 

 etc., the conditions existing in many of these beds of the Borrowdale 

 Series when the main changes took placp, did not materially differ 

 from those under which the Coniston Flags at Shap were meta- 

 morphosed near the granite, exactly similar solution of the titanic 

 acid and re-ci'ystallization as anatase having taken place in some of 

 these flags (Harker and Marr, Quart. Journ. Geol. Soc. vol. xlvii. 

 1891 ; also Hutchings, Geol. Mag. 1891, p. 462). 



Among the various changes which have taken place in the minerals 

 of these rocks, those which concern the felspars have the greatest 

 interest. The points involved ai'e, of course, the same whether we 

 study the felspars in the slates and other detrital rocks or in the 

 altered andesites, and what follows may be taken as applying equally 

 to both classes. 



The normal decomposition of the felspars of these rocks does not 

 seem, so far as can be made out, to lead to kaolinization.^ The three 

 main types of decay, sometimes singly, sometimes together, give rise 

 respectively to white mica, a verj^ pale chloritic mineral, and calcite. 

 The formation of white mica is very usual all over the district. 

 In many andesites and ashes the felspars, while retaining their 

 original outlines perfectly sharply, are internally quite full of mica 

 flakes, often of good size, hardly ever showing any sort of orientation 

 to the crystal-planes of the containing crystal, but occurring equally 

 in all directions and often as tangled clusters of flakes at all azimuths. 

 This lack of orientation of the mica formed in felspars appears to 

 be not unusual. I have noticed it in other rocks ; and it is pointed 

 out by Kosenbusch (Microscop, Physiog. der Gesteine, p. 23). 

 1 It is usually considered to be difficult, and indeed often impossible, to distinguish 



