Besides diamonds, there have been detected more than eighty species of minerals in the blue ground, the more common being magnetite, ilmenite, garnet, bright green ferriferous enstatite (bronzite), a hornblendic mineral closely resembling smaragdite, calc-spar, vermiculite, diallage, jeffreysite, mica, kyanite, augite, peridot, eclogite, iron pyrites, wollastonite, vaalite, zircon, chrome iron, rutile, corundum, apatite, olivine, sahlite, chromite, pseudobrookite, perofskite, biotite, and quartz. The blue ground does not show any signs of passing through great heat, as the fragments in the breccia are not fused at the edges. The eruptive force was probably steam or water-gas, acting under great pressure, but at no high temperature. According to Mr. Dunn, in the Kimberley Mine, at a depth of 120 feet, several small fresh-water shells were discovered in what appeared to be undisturbed material.
A selection of thin sections of some of these rocks and minerals, mounted as microscopic objects and viewed by polarised light, are not only of interest to the geologist, but are objects of great beauty.
The appearance of shale and fragments of other rocks testify that the _mélange_ has suffered no great heat in its present condition, and that it has been erupted from great depths by the agency of water vapour or some similar gas.
The rock outside the pipes and encasing them is called “reef.” Inside some of the mines occur large masses of “floating reef,” covering an area of several thousand square feet. In the De Beers Mine is what is called “the snake,” a dyke of igneous rock taking a serpentine course across the mine, and standing like a vein nearly vertical, varying in thickness from 2 to 7 feet. The main body of the blue ground is entirely analogous to the snake rock, naturally more decomposed, but in essential points the microscopic appearance of the blue ground and of the “snake” is in an extraordinary degree alike. Mr. Gardner Williams supposes that the “snake” is a younger eruptive formation coming from the same volcanic source as the blue ground. No diamonds have been found either in the “snake” or the floating reef. The ground, however, is generally richer in diamonds in the neighbourhood of the floating reef.