The portions
of Professor Lewis' manuscripts which, as explained in the Preface,
have not been printed, brought out more clearly one point in regard to
the origin of the diamond than is done in the two papers included in
this work, though I remember he laid stress on it in giving a summary
of the second paper at Manchester. This was the intimate connection of
peridotite (or serpentine) with the diamond, which he regarded as being
in the relation of cause and effect, at least in South Africa and in
Kentucky, where carbonadoes occur under similar conditions. This will
appear from the report of his paper printed in the •Geological
Magazine' (1888, pp. 129-131). It may suffice to quote the concluding
sentence : ' All the facts thus far collected indicate serpentine in
the form of a decomposed eruptive peridotite as the original matrix of
the diamond.' Professor Lewis, I believe, thought that, owing to the
basic nature of the peridotite, the carbon in the sedimentary rock with
which it came into contact was less likely to be oxidised than it would
be by more acid intrusives, but this opinion, so far as I know, is not
expressed in his published papers.
I
may add that in 1896 the workings in the I)e Beers Mine had reached a
depth of over 1,500 feet. They have ceased to work the diamond-bearing
rock by excavating it vertically downwards from the top of the 'pipe,'
but sink shafts through the ' country rock ' (shale, quartzite, and '
melaphyre '), from which they drive levels into the pipe itself,
removing its contents by means of these, as it were,
