TT. M. Hiitchings — Ash-slates of the Lake-District. 225 



simple deposit from solutions, exactly as albite, orthoclase, etc., are 

 known to be deposited on a larger scale in veins and cavities, 



Tliere are also, so far as I can make out, what certainly appear to 

 be orthoclase crystals in some of these rocks, which seem to require 

 another explanation. They are not in cavities, and for various reasons 

 they do not appear to be original crystals, but to be " regenerated " 

 like so many of the plagioclase crystals. I have come, after much 

 consideration of them, to regard them as not improbably pseudo- 

 morphs after former plagioclase, on the theory (or hypothesis) that 

 solutions containing silicate of potash have acted upon these former 

 felspars, removing soda and lime and replacing them with potash, 

 One such crystal observed, which optically fully corresponds to 

 orthoclase, contains a cavity lined with deposited rutile, evidently 

 formed at the time that solutions were strongly acting on the 

 original crystal. 



In some of the normal andesites the much-altered felspars full of 

 mica, chlorite or calcite, show in among these secondary products, 

 as before stated, more or less of clear glassy material. It is not an 

 uncommon thing to see this glassy felspar in considerable quantity 

 in long streaks and patches, and where the outline of the crystal is 

 fully preserved, as often is the case, to see that this felspar is 

 optically uniform all over the crystal, is quite free from twinning 

 and extinguishes perfectly parallel to the crystal-boundaries, or at 

 angles which point to orthoclase. In some of the rocks crystals are 

 frequent which are wholly glassy clear, save for trifling inclusions, 

 and behave as above described, occurring together with plagioclase 

 which is obviously regenerated. 



Such changes as I suggest may have taken place do not appear 

 at all unlikely under the conditions which we have reason to 

 infer obtained during the alterations these rocks have undergone, 

 and there appears to be good independent evidence that they have 

 been observed and chemically proved elsewhere. A leading authority 

 in this class of investigation appears to be Lemberg. I have not 

 at present an opportunity of consulting his original papers, but in 

 Koth (Chemische Geologic, vol. i.) among other abstracts from and 

 references to Lemberg's work are certain facts which bear very 

 directly on the point in questions. 



Thus, Lemberg analyzed oligoclase from the tourmaline-granite of 

 Monte Mulatta, and its green alteration-products in four examples, 

 a, h, G, d. The alkalies were as follows : — 



water is also taken up at the satne time. 



Other analyses of Lemberg's are also given, showing similar 

 changes in oligoclase, some chloritic mineral being apparently formed, 

 with a simultaneous removal of soda and increase of potash. An 

 analysis of labradorite is also given, together with that of its altera- 

 tion-product, showing — 



DECADE III. — VOL. IX. NO. V. 15 



