Petrograpkical Differentiation of certain dykes. 155 
Magnetite, ave. diameter 0147 mm 
Porphyrinic crystals. 
Pyroxene, largest polysomatic grain 1.120 mm 
Feldspar, largest lath-shaped crystal 050 x .560 mm 
II Pyroxene, approx. ave. diameter 840 mm 
Feldspar, " " size 560 X .056 mm 
Magnetite, " " diameter 126 mm 
III Pyroxene in polysomatic masses, ave. diameter. . .. .v'.OOO mm 
Feldspar, lath-shaped crystals, largest 350 X .100 mm 
Magnetite, few scattered grains, ave. diam 700 mm 
Quartz, ave. diameter 050 mm 
IV Pyroxene, much altered to hornblende, larger gr.. 1.000 mm 
Feldspar, ave. size of larger grains 2.000 mm 
Magnetite, few large grains, diam 700 mm 
Quartz, larger grains 840 mm 
Structural variation. — The specimen taken at the contact with 
the dyke walls (I) appears in section as a porphyrite. The 
ground mass is a fine ophitic felt work of plagioclase and greenish 
yellow pyroxene, with viridite thickly studded with granules of 
magnetite, all of the latter belonging probably to the final con- 
solidation of the magma. The porplryritic constituents are in the 
order of their generation (1) Plagioclase in lath-shaped crystals, 
Ji 
1 1, 
1 
- 1 . - ■ 
;&«ti 
Fig. 1. 
Slop [gland dyke. — Section of dyke-rock at contact with dyke wall. 
X38. a. polysomatic augite; 1'. plagioclase. Illustrating porphyrite 
structure. f 
