46 DE. J. W. EVANS ON A MONCHIQFITE FEOM [Feb. I9OI, 



In the isotropic material filling the interstices between the 

 coloured minerals, the arrangement of the inclusions is somewhat 

 similar to that in the nepheline, though rather less regular in 

 character ; but groups of parallel inclusions at right angles one to 

 the other may be distinguished. The inclusions in the colourless 

 spherical spaces also usually show two systems of linear arrange- 

 ment, at right angles one to the other. In one of the colourless 

 spaces that appears to be isotropic, even in convergent light, 

 numerous hairlike needles may be observed ; the most conspicuous 

 of these lie approximately in the plane of the section, and show 

 an orientation in three directions meeting at angles of 60". Others 

 are inclined at a considerable angle to the section, and these also 

 probably lie in symmetrical directions. In another case minute 

 ill-defined inclusions are arranged in six directions, meeting each 

 other at angles of 30°. 



The rounded and irregular inclusions, though often occurring in 

 straight lines, are also found in irregular curves, which are some- 

 times continuous with a rectilinear arrangement. It is doubtful 

 whether any of the acicular inclusions except the apatites pene- 

 trate the coloured minerals. Where they seem to do so it is, I 

 believe, an optical illusion due to superposition. 



The true nature of the majority of these inclusions must be left 

 undecided. Many are, as I have said, apatites ; others are liquid-, 

 or, in a few cases, gas-cavities ; and some may be minute segirines 

 or hornblendes which are too small for determination. 



Under a high power indications of an imperfect cleavage are 

 clearly visible in the isotropic matrix, especially in the circular spaces 

 (see PL II, fig. 2). Generally two cleavages are seen, perpen- 

 dicular one to the other, though the inclination of the lines where they 

 meet the microscope-section sometimes deviates from a right angle. 

 The cleavage-cracks are usually parallel to the principal directions 

 followed by the inclusions ; they sometimes pass into curves at their 

 extremities. In the interstices between the older minerals, faint 

 traces of a similar rectangular cleavage are occasionally seen. 



IV. Chemical Analyses. 



The rock was crushed, and the powder separated by means of the 

 double iodide of mercury and potassium. The material collected 

 with an average specific gravity of a little over 2-2, was found to 

 consist mainly of an isotropic substance ; a few small prisms of 

 ferromagnesian silicates were seen to be embedded in it, and no 

 doubt a little nepheline and felspar were also present. 



This separated material was treated with acid, and the dissolved 

 bases were determined. 



The following table summarizes the results of this analysis, and 

 the inferences that may be drawn as to the composition of the 

 isotropic groundmass. Other analyses of similar character are 

 appended, as well as the percentage composition of typical nepheline 



