ON THE HISTORY OF THE DIAMOND 9
ing to 
and 
are unusually well marked, that
according to
being often in straight lines almost
as
good as in felspar. Olivine is known to occur in rocks in three
different habits, according as the rock is granular, porphyritic, or a
crystalline schist, as Eosenbusch1 has well described. The
rock in question contains olivine, with the form belonging to
porphyritic rocks. The distinct crystalline shape has been altered by
subsequent corrosion. Olivine is the main ingredient of the rock, all
other minerals being accessory.
Of
these the principal are enstatite and biotite. The enstatite is clear,
not pleochroic, and is readily determined by its parallel extinction ;
the biotite is in crystals, which are usually rounded by corrosion, as
is indicated by the black rim which often surrounds them. Similar rims
are well known around the biotite of many andesites, trachytes, and
related rocks. The black rim is shown to be made of a mixture of
magnetite and augite, and is thickest when the ground mass is
holocrystalline, and thinnest when most glassy. The very thin rims here
argue a glassy base. The biotite, as in peridotites and
nepheline-basalt, is usually twinned after the law of Tschermak.
Serpentines with garnet occur as dykes in the Yaal River. Dr. J. Shaw speaks of serpentines,2 and Professor T. B. Jones of garnets, in Yaal River gravels.3
Dr. Shaw ' also states that dykes cross the Yaal River and make rapids.
Garnets abound in these gravels, also tourmalines, talc, and mica.
