484 F. RTJTLEY ON SOME PECULIAKITIES IN THE 



from simply striated areas, and which in other instances may divide 

 areas all of which show crossed striae, although those striations do not 

 run at right angles. Attention has also been directed to the oc- 

 currence of transverse bands crossing the longitudinal bands in plagio- 

 clase ; and I now leave it for others to decide how far such phe- 

 nomena strengthen or weaken the value of those tests by striation 

 on which microscopic petrologists have, at present, to rely so much. 

 I am inclined to think that examples such as those now brought 

 forward occur rather exceptionally, and that our belief in the exist- 

 ing method of microscopically distinguishing orthoclase from the 

 triclinic felspars need in no way be shaken, as a rule. Still it is 

 well to bear in mind the fact that exceptional cases may occur 

 more frequently than we anticipate, and it seems quite possible that 

 by careful examination of these cases we may acquire a few addi- 

 tional scraps of knowledge respecting the structure of crystals and 

 the value of the microscopical tests which we are in the habit of 

 using. 



On Plate XXXV. a few other examples are given, illustrative of 

 the points already discussed in this paper. 



From what has already been observed and recorded by the best 

 microscopic petrologists of the day it seems to be an established fact 

 that orthoclase in thin section under the microscope polarizes either 

 in uniform sheets of colour or in sheets of varying colour, the varia- 

 tion being due to differences of thickness in different parts of the 

 section — that crystals of orthoclase, when twinned on the Carlsbad 

 type, polarize in complementary colours on opposite sides of the 

 twinning plane, owing to a difference in the direction of the elastici- 

 ties in the component halves of the twin — that some massive, 

 cleavable kinds of orthoclase, such as those from certain localities in 

 Norway, Sweden, and elsewhere, exhibit a cross-hatched rectangular 

 banding, often in strong colours, when seen by polarized light — that 

 the triclinic felspars may under similar circumstances, when cut 

 parallel to the twinning-planes, exhibit only monochromatic polari- 

 zation, under which conditions they may be confounded with sections 

 of orthoclase — -that it is a doubtful, although not an impossible thing, 

 that a crystal of triclinic felspar may be only once twinned and thus 

 again simulate orthoclase — that crystals of triclinic felspars are almost 

 invariably many times twinned, thus giving rise to parallel banding of 

 different tints when cut transversely or obliquely to the twinning 

 planes, the bands having in the former case sharply defined margins, 

 while in the latter instance they may soften into one another on revo- 

 lution of one of the Kicols — that some of these bands in triclinic fel- 

 spars occasionally end abruptly, and that in altered conditions either 

 of orthoclase or plagioclase a more or less granular structure super- 

 venes, which often renders it difficult or impossible to distinguish 

 between them. So far as my limited experience goes, I can indorse 

 these statements, but I am not aware that any exceptional instances 

 such as those which I have pointed out, have yet been recorded. 



There remains one more point to be mentioned with regard to the 

 banding in plagioclase crystals under polarized light ; and to this I do 



