RETORT ON THE PETROLOGY OF THE ROCKS OF ST. PAUL. 9 



than those in the dunite, are perfectly colourless and transparent, their surfaces presenting 

 a certain rough appearance, which has long since been known to be one of the microscopic 

 characteristics of this mineral. In polarised light the grains are clearly individualised, 

 and the rock presents the phenomena of aggregate polarisation, each grain of olivine 

 exhibiting the brilliant tints peculiar to thin sections of this mineral. Besides the olivine 

 of the ground-mass there are also scattered grains of the same mineral, whose diameters 

 sometimes reach 0"5 mm., thus giving the rock a microporphyritic structure, as already 

 mentioned. These larger grains are mostly rounded or ellipsoidal ; I have never ob- 

 served any whose contours recalled the crystalline form of olivine. They are colour- 

 less and somewhat rough on their surfaces. In polarised light they assume very brilliant 

 tints ; they contain what would seem to be liquid enclosures. They are frequently fur- 

 rowed by cracks or joints, and in some cases lines, distinctly parallel, corresponding to 

 the brachydiagonal cleavage, are distinguishable ; but these cleavage lines are never either 

 so distinct or so numerous as the fibres in the sections of enstatite, which will be described 

 later on. These lines are occasionally crossed by others, perpendicular to them and parallel 

 to one another. These must correspond to the macrodiagonal cleavage, but the latter 

 cleavage is not so distinctly marked as the former. It occasionally happens, however, 

 that no cleavage is observed in the section. When the lines of cleavage are sufficiently 

 marked, and when the extinctions with polarised light are sought for, the more or less 

 elongated sections are found to possess the optical properties of crystals belonging to 

 the prismatic system ; these sections are extinguished parallel and perpendicular to the 

 direction of the cleavages. The large and somewhat elliptical sections of olivine are 

 occasionally formed by the agglomeration of a number of grains, and when examined 

 in polarised bght present a very brilliant mosaic. They are most frequently met 

 with when the ground-mass consists of veiy minute grains, and assumes the banded 

 structure. 



After the olivine the most frequent ingredient is chromite. The grains of this 

 mineral are seen to glitter on fresh or polished surfaces when the specimens are examined 

 macroscopically. They do not exceed 0'5 mm. in any direction, generally vary from 

 O'l mm. to 0"05 mm., and are mostly opaque, save where sufficiently thinned by 

 polishing. In the latter case they are transparent, yellow, or chestnut-brown, rarely of 

 a faint green ; thus bearing no small resemblance to certain lamellae of biotite (figs. 1 

 and 3). On a cursory examination these transparent sections would probably be classed 

 as hornblende or biotite. When examined more closely the resemblance is found to 

 be merely apparent, for there are no traces of cleavage or of lamellar structure. Di- 

 chroism is entirely absent, and all the transparent sections of any size remain persistently 

 dark during a complete rotation between crossed Nicols. This last feature shows that 

 they belong to some mineral of the cubic system. The irregular outlines of these 

 sections, however, might at first sight make it doubtful whether they could be referred 



B 



