and Igneous Rocks of Ensay. 113 



The pinite schists, so far as concerns the principal con- 

 stituent, are evidently pseud omorphic, and it is probable 

 that they are so after magnesia-mica mainly. In the 

 examples which I have examined microscopically I have 

 found two kinds of mica, in addition to the pinite material, 

 One is a brown magnesia iron-mica, and the other an alkali- 

 mica. The latter is in many cases, but not in all, greatest 

 in amount, and is evidently in some instances of secondary 

 origin. The analysis of the pinite schist shows that it is not 

 far removed from the composition of the pure pinite mineral. 

 The pinite in these schists when -seen under the micro- 

 scope and by polarised light, especially in slices across the 

 foliations of the rock, has a "meshed" appearance, resembling 

 that of serpentine, to which mineral it has also a resemblance 

 macroscopically when the rock is examined on a cross 

 fracture. In places these schists contain masses of alkali- 

 mica flakes, and more rarely colourless, divergent, or fan- 

 shaped clusters of talc plates. 



On the whole, I can see no more probable conclusion than 

 that these pinite schists are pseudomorphic alterations of 

 a schist rich in magnesia-mica. The conversion of magnesia- 

 mica to pinite has been recorded by Blum, Dana, and Vom 

 Rath,* and it may have been the case here on a large scale. 

 Yet I must notice, as not falling in with this view, that I 

 have not met with a single instance in all the thin slices 

 which I have prepared and examined wherein magnesia- 

 mica has been partly, or indeed, so far as I could see, entirely 

 converted into anything else than chlorite. That is to say, 

 in all the slices I have referred to there have been numerous 

 individuals of magnesia-mica, either intact or partly 

 chloritised ; others wholly converted into chlorite ; but not 

 one single flake which showed a partial conversion into 

 pinite, such as is so commonly the case in the felspars. 

 Therefore the complete proof of the origin of the pinite is 

 still wanting. 



The gneissic schists are characterised by the pre- 

 valence of a monoclinic potassa felspar, often in porphy- 

 ritic crystals, or that mode of occurrence which gives occa- 

 sion to the term " Augen-gneiss" of the German writers. 

 This orthoclase has been the first formed of the felspars in 



* Quoted by Both, Allgemeine Chemische Geologie, Vol. I,, p. 332 ; Blum, 

 Pseudom, I., 79, and in., 142 ; Dana, Amer. J. of Sc. (3), 8, 449, 1874 ; Vom 

 Bath, Zs. Geol, Ges., 27, 382, 1875. 



I 



