522 THE CONTINENTAL ORIGIN OF FI.TI, II., 



lower than that of Canada l)al8am, and are ver}" faintly doubly 

 refracting. The amount is small, and the properties obscure. 



The felspars of the first generation vary in size up to 2 35 x 

 1-25 mm. They are very clear and free from decomposition. In 

 many cases they are crowded with inclusions which are usually 

 more or less zonall}^ arranged. In some sections "dusty" inclu- 

 sions are so abundant as to render the felspars almost opaque. 

 These inclusions are described later. The zoning produced by 

 isomorphous layers of felspars of varying composition is strongly 

 marked by differences of refractive index in ordinary light, and 

 by differences of double refraction in polarised light, especially 

 in sections parallel to (010). The refractive index is in all 

 cases markedly higher than that of Canada balsam. The 

 cleavages parallel to -{ 010 } and -| 001 [> are sharply defined, 

 while that parallel to ^ 110 )- is marked by strong cracks. 



Between crossed nicols almost all the sections exhibit albite 

 twinning occasionally combined with that after Carlsbad and 

 pericline laws. Zoning, expressed by differences in double 

 refraction, is less marked in sections perpendicular to < 010 }- 

 than it is in some of the other rocks described: but, as stated 

 above, it is a striking feature in sections parallel to <| 010 [> . 



Sections in the zone perpendicular to <{ 010 [- give symmetrical 

 extinctions up to a maximum of 32'-^, with a difference of 15° 

 between the extinctions in the two parts of a Carlsbad twin. 

 Good sections parallel to -J 010 [- give an extinction of- 11" for 

 the peripheral, and - 24'-' for the central portions. These 

 measurements indicate that the external zones consist of a basic 

 andesine, while the central kernel is a basic labradorite. 



The inclusions in the felspars may be divided into glassy and 

 lithoidal, gaseous, individualised and "dusty." 



The glass}^ and lithoidal inclusions are very varial^le in size 

 and shape. As a rule they are round or irregular, but occa- 

 sionally they approach the form of negative crystals. The glass 

 is yellowish or brownish in colour, while the lithoidal portions 

 contain thread-like microlites, and pass insensibly into those 

 described as dusty. 



