384 O. E. LE ROY — GI^:OLOGY OE RIGAUD MOUXTAIX, CANADA 



zone being quite clear. The products are numerous shreds of kaolin, 

 with an occasional minute grain of epidote or calcite. A mechanical 

 separation by means of Thoulet's solution resulted in the oligoclase 

 falling at a specific gravity of 2.62, the greater part of the microperthite 

 at 2.56, and the remainder, which is probably almost pure orthoclase, at 

 2.52. 



The microperthite has a poor form, occurring in large irregular or thick 

 lath shaped interlocking individuals, with smaller grains between. The 

 only approach to idiomorj)l>ism is when the individual is in contact with 

 the quartz. In this case there is a partial development of some of the 

 crystal faces. The cleavage parallel to c (010) is usually distinct; that 

 parallel to h (001) less so. 



The hornblende is the common green variety, with an extinction angle 

 of 20 degrees. The pleochroism is strong, the absorption colors ranging 

 from deep green to pale yellow tone. Though usually fresh, in a few 

 instances it is seen altering to chlorite either along the cleavage planes 

 or zonally from the interior ([)late 33, fii<ure 2). The greater part of the 

 mineral follows the ordinary law, cr3'stallizing out before the feldspar, and 

 is generall}' intergrown with augite and biotite. It is rarely idiomorphic, 

 occurring in prismatic sections with terminal faces, poorly developed, 

 in rounded basal plates and irregular grains. The larger individuals 

 frequently exhibit a poikilitic structure, inclosing differently oriented 

 grains of feldspar. The borders of the individuals are seldom smooth, 

 but send out little extensions, which, penetrating the feldspar, in many 

 cases develop into a skeleton crystal and form a graphic intergrowth 

 with the latter mineral. Figure 2, plate 33, shows this structure; the 

 skeleton crystal is in a feldspar individual, but joined to and in optical 

 continuity with the parent grain. At other times the skeleton is not so 

 connected, and then usuall}'^ presents a delicate lattice like structure, 

 made up of slender prismatic rods connected at intervals by narrow 

 transverse sections. The most striking structure, however, is when the 

 hornblende occurs in forms strongly allotriomorphic toward the feld- 

 spar. In this case it fills triangular and polygonal interstices between 

 the feldspar individuals, and is often associated with quartz. Figure 1, 

 plate 34, shows this excellently, though for purposes of better illustra- 

 tion biotite is reproduced instead of hornblende. Ramsay and Hack- 

 man* have described a similar structure in the nepheline syenite of 

 Pontelitschorr, in which aegerine assumes the form superinduced by the 

 nepheline and feldspar, and in the same rock a poikilitic intergrowth of 

 arfedsonite and segerine occurs with the colorless constituents. 



Augite, when present, almost invariably occupies the center of pris- 



*Das nephelinesyenitgebiet auf der Halbinsel Kola, Fennia 11, no. 2, p. 127. 



