KiNAHAN — On Microseopieal Structure of JRocfcs. 101 



this felspar are so opaque that their structure cannot be properly seen ; 

 this is probably due to the slice not being cut thin enough. Enough, 

 hovever, can be seen to know that the felspar is similar to those pre- 

 viously described and supposed to be adularia. 



Tridhvic felspar. This felspar, when the rock was examined in 

 the field, could not be distinguished from the preceding. In the 

 portions of the crystals associated with the adularia (?) the lines of 

 structure are straight and parallel, and the play of colour regular, but 

 this is not the case in the individual crystals and masses. In one 

 crystal, with a power of 196, there are straight, sharp, but not per- 

 fectly parallel, lines, while the play of colours is faint. Under 

 a power of 396 the straight sharp lines seem to be due to cleavage 

 or shrinkage lines, and are now filled by thin films of a black 

 mineral ; they probably are accidental adjimcts to the crystal. In a 

 second mass of triclinic felspar we find, with a power of 196, that the 

 layers of structui'e are not parallel, some being inclined to be lenticular, 

 the lines of green and white being short, of different thicknesses, or 

 joining into one another ; yet the play of colour is very regular. 



H. I. A. PIIOC, SER. II., A'OL. II., SCIF.XC'E. 



