PECULIARITIES IN THE MICROSCOPIC STRUCTUKE OF EELSPARS. 479 



35. Notes on some Peculiarities in the Microscopic Structure of 

 Felspars. By Frank Rutley, Esq., F.G.S. (Read May 26, 1875.) 



[Plates XXIII. & XXIV.] 



As the different striations presented by various sections of felspars 

 when viewed by polarized light under the microscope are, I believe, 

 regarded by many observers as very strong if not absolutely con- 

 clusive evidence of the crystalline system to which those felspars 

 belong, and since determinations upon this point are matters of the 

 utmost importance to the petrologist, I have endeavoured to ascer- 

 tain, so far as I am able, the real value of some of these phenomena 

 as tests in microscopic inquiry. In 1869 Kreischer observed a 

 series of striae in sections of yellowish brown pegmatolite and in a 

 flesh-coloured felspar from Arendal, which crossed one another at 

 right angles — the one system lying horizontally at right angles to 

 the principal axis, whilst the other followed the direction of the 

 basal plane ; and he noticed these striations in sections taken parallel 

 both to the basal and to the clinodigonal cleavages*. 



Stelzner suggested that this appearance was due to the mineral 

 being composed not merely of coarse lamellae, but of an intergrowth 

 of two systems of finer ones, one system lying parallel to the ortho- 

 pinakoid, the other parallel to the clinopinakoid, which traversing 

 one another in opposite directions were subdivided into rod-like 

 forms f. These striations have subsequently from time to time 

 attracted the notice of different observers ; and whenever they occur 

 in a felspar it is regarded as orthoclase. I doubt whether any 

 perfectly satisfactory explanation of this phenomenon has yet been 

 offered ; and it seems to me better to study the effects before attempt- 

 ing to deal with the cause. A careful examination of microscopic 

 appearances will probably do much towards teaching us the real 

 character of these markings, and may at the same time help, to some 

 small extent, to demonstrate those points of structure which con- 

 stitute the groundwork of crystal architecture. The following notes 

 refer to various peculiarities of structure which I have observed in 

 different felspars ; and it is my object to show how much credence can 

 be placed in certain microscopic appearances, in the discrimination 

 of orthoclastic and plagioclastic felspars. 



Fig. 1, Plate XXIII. represents part of a section of orthoclase from 

 Arendal, seen by polarized light under a magnifying power of 50. 

 In this the cross-hatched striation is well shown ; but the most pe- 

 culiar feature consists in the occurrence, in this portion of the section, 

 of some coarser lamellar particles or crystals which follow two 

 definite directions, these directions intersecting at an oblique angle, 

 the alternate angles measuring about 75° and 105°. The directions 

 assumed by these plates or crystals appear to lie in positions inter- 



* Neues Jahrb. f. Mineral. 1869. p. 208. 

 t Berg- u. hiittenmann. Zeit, xxix. 150. 

 Q, J. G. S. No. 123. 2 k 



