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52 THE MATRIX OF THE DIAMOND
phyritic
or the brecciated structure happens to predominate in the section under
examination, the observer varies in his opinion as to whether the rock
is a lava or a tuff.
Cohen,
Hudleston, Moulle, and others, believed it to be an igneous tuff, while
Dunn, and Maskelyne and Flight considered it to be an eruptive
gabbro-like rock.
When
comparatively free from enclosures the porphyritic structure may be as
distinctly shown as in a basalt. The greater mass of the porphyritic
crystals (the' einspreng-linge ') are the olivines, which often have
distinct crystalline form. These are, as already stated, more
frequently rounded and like the olivines in basalts, and may show the
action of a corrosive magma. Biotite, bronzite, garnet and other
substances also form comparatively large crystals, usually rounded,
and, with the olivines, lie separately in a more or less isotropic
ground-mass made mainly of a serpen-tinous mineral. The rock therefore
is not holocrystalline, but belongs to the class of volcanic or
effusive rocks (' erguss-gesteine'), characterised by idiomorphic
porphyritic crystals floating in a ground-mass.
That the rock was a true igneous lava, and not a mud or ash, is indicated by the following facts :—
1. The minerals and their associations are those character-
istic of eruptive ultra-basic rocks.
2. The porphyritic crystals are idiomorphic as in volcanic
rocks. 3. The corrosion cavities (' einbuchtungen') in the porphyritic crystals are due to solution by the hot magma.
4. The character of the bronzite and diopside is similar
to that in meteorites and eruptive rocks, but not in metamorphic or plutonic rocks.
5. The occurrence of a ground-mass and of traces of glass.
6. The traces of a second generation of minerals (pyroxene'?)
in the ground-mass.
7. The occurrence of fragmentary enclosures of the adjoining
rock and of deep-seated rocks, and the evidence of alteration by heat which these enclosures exhibit.
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