54 THE MATRIX OF THE DIAMOND
not due to pressure. The brecciated structure, therefore, whatever its cause, is not due to pressure.
Several
causes may produce brecciation in a volcanic rock. It may bedue :—1. To
the rapidcooling and consequent shrinking and cracking of a fluid lava
; 2. To the contact of the moving lava with the adjoining rock which is
carried along with it and broken up to form a so-called friction
breccia ('reibungsbreccia'); 3. To subsequent explosions and movements
in the crater of the volcano. It is quite possible that in the present
instance all three of these causes may have operated.
As to the first cause, the words of Dr. "Wadsworth,1
if applicable, seem prophetic. After explaining the brecciated
character of meteorites as due to rapid crystallisation, he says : ' If
we could find rapidly cooled, unaltered terrestrial peridotic rocks, I
should expect to find in them the chon-dritic structure, -the same as
the Estherville meteorite possesses the structure of an unaltered
terrestrial peridotite, and the meteoric pallasites possess that of the
terrestrial ones.' Roth 2 speaks of the formation of lavas and slags which fall apart on cooling.
The
second process, which causes a ' friction breccia,' is well known in
many volcanic districts. It is seen on approaching the vent of the
volcano. Naumann3 has given some excellent examples of this
in Saxony, where in the border between porphyry and shales, great
masses of the breccia, the so-called ' Brockengesteine ' is formed, and
the mass of the enclosed slates may predominate over the mass of the
porphyry.
The
third process, the result of successive explosions in the same crater,
is also common in volcanic districts. In this case, not only are
fragments of the adjoining rock enclosed in the lava, but the older
and hardened magma itself is broken up and imbedded afresh in the new
erup-
1 Lithological Studies, 1884, p. 111.
2 Allg. unci chem. Geol. Berlin, ii. 1883, p. 207.
3 Lehrbuch der Gcognosie, Leipzig, i. 1858, p. 917.
