THE MATRIX OF THE DIAMOND 53
8. The traces of a fluidal structure shown on polished
specimens.
9.
The identity of the rock with one in Kentucky, which is a true eruptive
dyke, and with others in the Vaal River, which also form dykes.
That
it is not a tuff is shown by the entire absence of stratification, of
clayey or ashy bands, of water-worn fragments, of cementing material,
and by its geological position : not as an overflow mud deposit, but in
the very neck of a volcano.1
The
brecciated structure of the rock is shown not only in the abundance of
angular fragments of shale it contains, but also in the broken and
angular character of many of the porphyritic crystals in it, and the
mechanical manner in which they seem to have been sometimes pushed
together.
Yet
the structure is not a purely mechanical one, such as is the case with
certain ancient rocks 'which have been subjected to heavy pressure. A
so-called mechanical-por-phyritic structure is well known in
deep-seated peridotites,2 being particularly well marked in the variety named olivin-fels. These, as Brogger 3
and Eeusch have shown in Norway, are often so crushed as to resemble a
sandstone. Here, however, under the microscope the so-called '
cata-clastic' structure,4 or 'mortel-structur,'5
whereby the outer portions of the crystals are smashed into a
mosaiclike band (as in ' augengneiss,' &c.) can always be seen. No
traces of this cataclastic structure occur in the Kimberley peridotite.
The olivines have no mosaic borders or ends, no ' eyes' of them are
made, and their rounded form is certainly
