316 W. M. Hutchings — A Contact- Rock from 8hap. 



passing down to a degree of fineness which is only just capable of 

 definition with moderately high powers and good illumination, and 

 there are, again, grains in which it is not seen at all. The same is 

 true of the grains of more moderate size, and among these some few 

 of them which are not striated are single twins, extinguishing quite 

 'parallel and safely referable to orthoclase. 



Among the larger grains a good number have cleavage distinctly 

 developed, and by patiently searching among these, cases may be 

 found in which there is emergence of a positive bisectrix nearly 

 central, with an extinction-angle, measured against the well-marked 

 cleavage, of 10° to 12° — very often 12° exactly. 



These characteristics — the minute lamellation passing down into 

 such a degree of fineness that it cannot any longer be made out, and 

 the extinction-angle of about 12° on m flakes — seem to indicate that 

 we are dealing with a felspar similar in nature to that which occurs 

 in pegmatitic veins at Frederiksviirn, in connection with the so- 

 called soda-orthoclase, with which it appears to be intimately 

 related by passages from one to the other, with a similar chemical 

 composition in both cases. 



Brogger has concluded that this lamellar felspar at Frederiksvarn 

 is a microscopic intergrowth of orthoclase and albite — a micro- 

 perthite — passing down into a sub-microscopic degree of intergrowth 

 which cannot be resolved by the microscope, and which he designates 

 as cryptoperthite. 



The analysis of the felspar from Frederiksvarn shows about 

 7"5 per cent, of potash to 7 per cent, of soda, and Rosenbusch points 

 out that the extinction-angle of 12° corresponds approximately to 

 the medium angle between orthoclase on the one hand and albite on 

 the other. 



As a further test of the probable nature of this felspar in the 

 Shap rock, T have recently carefully analysed an average sample of 

 a large hand-specimen, with the following result : — 



per cent. 



Silica 53-10 



Alumina 23-60 



Ferric oxide 8-50 Ferrous oxide not determined. 



Lime 3-16 



Magnesia 1-62 



Potash 4-67 



I Soda 4-05 



Water 0-55 



99-25 

 It will be seen that the relative proportions of potash and soda 

 do not vary appreciably from those in the Frederiksvarn felspar, 

 and as the alkali-bearing minerals present in this rock, other than 

 felspar, are quite insignificant in amount (merely the very small 

 proportion of micas) we may look on all this alkali as practically 

 representing the felspar. This makes the resemblance to the 

 Frederiksvarn mineral satisfactorily complete, and it is an in- 

 teresting thing to observe its occurrence, as a new formation, in 

 a rock altered by contact-action. The case is of all the more 



