Speight. — On a Doleritic Dyke at Dyer's Pass. 413 



the crystal. The extiuction-aiigle was 47°. The section 

 would probably be parallel to OP. However, many crystals 

 exhibited no cleavage-cracks at all. Crystals show both binary 

 and polysynthetic twinning. In one case (PI. XLIXa., fig. 4) 

 a twinned band was observed to be faulted, but no disruption 

 appeared in the crystal, the part which should have been 

 occupied by the faulted lamella being occupied by the ad- 

 jacent one. In one case a part of a crystal polysynthetically 

 twinned was surrounded by a portion free from twinning. 



Olivine. — Crystals of olivine occur throughout the rock. 

 Occasionally they are unaltered, but in the great majority of 

 cases cracks occur with alteration proceeding from them. 

 The product of decomposition is limonite. In some cases the 

 alteration has proceeded so far that only a pseudomorph of 

 limonite remains. 



Magnetite. — Grains of magnetite occur throughout the 

 rock, both in the ground-mass and porphyritically, but they 

 graduate into one another. In many cases they are included 

 in the augite and olivine, thus showing it was the first 

 mineral in order of production. 



Geound-biass. — The ground-mass consists of a holocrystai- 

 line aggregate of feldspar microliths and augite and magnetite 

 grains. The feldspar was difficult to determine, since the small 

 crystals nearly always exhibited undulose extinction. By the 

 method of extinction of twin lamellse it proves to be labra- 

 dorite or anorthite, since in most of the cases observed the 

 angle w^as over 20°, and in some few over 40°. However, the 

 method of extinction, with the length of the microliths, gave 

 contradictory results, as nearly all extinguished in the length 

 of the microlith, or at very small angles from it. This would 

 point to oligoclase. This is not very uncommon in the 

 ground-mass of a rock of the basic series, but the first method 

 would probably be more reliable, and the conclusion to be 

 arrived at is that it is probably labradorite or anorthite. 

 The augite and magnetite showed no remarkable structure. 

 There was no glass apparent. 



General Conclusions. 

 The foregoing description shows that the rock must be 

 classified as an oliviue-dolerite, or, if olivine is to be con- 

 sidered as an essential mineral of the basalt group, it would 

 be called simply a dolerite. The holocrystalline nature of 

 the rock is probably due to slow cooling in a fissure at a 

 slight depth beneath the surface, but not sufficient to allow 

 large crystals to form. 



