and their Contact Zones. 31 



lamellse, whose intersection is nearly perpendicular, and in 

 which the alternate lamellae are twins. This structure of 

 microcline produces a highly characteristic cross-banded 

 appearance by polarised light. 



The observations which I have made on these triclinic felspars 

 entirely accord with the above, and although a quantitative 

 analysis of the rock is still required, I feel that I may with 

 some little confidence refer these felspars, as I have done, to 

 microcline. I shall have to point out later in this essay 

 that the felspar composing one of the contact schists is 

 without doubt a triclinic potash felspar, and that it agrees 

 optically with the above requirements of microcline ; it also 

 agrees in its character with the felspars, which are com- 

 ponents of those rocks which adjoin the schist* 



The inclusions in these felspars are few in number as in 

 species. Apatite in small colourless prisms and needles is 

 the most frequent. In some few cases I have observed 

 microscopic flakes of magnesia mica, or of amphibole, and 

 also minute, colourless, rather stout prisms, whose termina- 

 tions suggest a monoclinic habit. Very rarely I have 

 observed what appear to be octahedral crystals of magne- 

 tite. Besides these, which may be regarded as original 

 inclusions, there are also frequently flakes of chlorite and 

 patches of iron ores, evidently the result of alteration and 

 infiltration. 



The alterations which these felspars have undergone are 

 of two kinds, of equal frequency of occurrence — kaolinisation 

 and micacisation. In both cases the change has mostly 

 commenced from the centre and extended outwards along 

 the cleavage planes. But occasionally I have observed that 

 some lamellaB have been more affected than others. When 

 the felspar has been kaolinisecl it becomes grey and cloudy, 

 and loses its characteristic chromatic polarisation, without 



* I may note that I have, as yet, failed to isolate these felspars. As an 

 instance, I may take the following : — I obtained a fragment from the perfect 

 cleavage of a crystal apparently of this felspar in one of the coarse-grained 

 samples which had afforded me a characteristic thin slice. From part of 

 this I succeeded in preparing a microscopic object ; the other part I examined 

 before the blowpipe for potassium. The microscopic examination proved it 

 to be not striated, and that the direction of obscuration was parallel, or 



nearly so, with the edge o — ii (supposing it to be triclinic). The portion 

 examined chemically gave only a very slight precipitate of chloride of 

 platinum and potassium. Before the blowpipe I could not detect 

 potassium with any certainty by observations of the flame reaction through 

 cobalt glass. The above experiments point, I think, to a simple crystal 

 of oligoclase. 



