2*24 
THE MEDITERRANEAN NATURALIST 
birds have been exterminated jn very recent 
years, but other facts point to. a different conclu- 
sion. Traditions seems to 'Show, according to Mr. 
F. W. Hutton, that the Moa became extinct in 
the North Island soon after the arrival of the 
Maoris in New Zealand — that is, not less than 
400 to 500 years ago — and in the South Island 
about a hundred years later. The fresh-appearing 
skin and ligaments are supposed to have been 
preserved by unusually favorable conditions. 
Geology of the Nile Valley. 
At a recent meeting of the Geological Society of 
London a communication was read by Mr. Nor- 
man Tate from Messrs E. A. Johnson Pasha and 
H. Droop. Richmond entitled “Notes on the geo- 
logy of the Nile Valley” in the course of v, hick 
the authors state that the rocks on either side of 
the Nile from Cairo to Esnel are chiefly Eocene ; 
south of this they are sandstones, probably Carbo- 
niferous, and they yield indications of coal. They 
extend to Assouan, where they meet the granite 
and basalt of that region. 
A few miles to the south, sandstone again ap- 
pears and save that it is occasionally broken by 
granitic dykes it extends to Wady Haifa. 
The granite is intrusive into, and alters the 
sandstone, whilst the latter reposes upon the 
basalt and in some cases was deposited upon up- 
standing basaltic masses. To the east of Minieh, 
west of Assioutt unmistakable lavas appear. The 
paper concludes with a brief description of the 
minerals found in the sedimentary strata, and of 
the principal faults that occur in the district. 
The Relationship of the Structure of 
Rocks to the conditions of their 
Formation. 
By H. J. Johnston Lavis, m.d. 
Of course the sheet of igneous magma may so- 
lidify at any part of its journey towards the sur- 
facejn consequence of— 
(a) Loss of heat from conduction away by the 
surrounding rocks. 
(b) Raising the acquired water to the mean tem- 
perature of the solution of silicates in which it 
is dissolved. 
(c) By loss of heat in consequence of expansion 
with vesiculation during the extension of the 
fissure. 
(d) By gradul escape of water in the form of steam 
or vapour through fissures so supplying fumaruli. 
( e ) By convection currents of waters forming 
Geysers or thermo-mineral springs. 
It is a common fact that the water-bearing 
qualities of different rock strata are widely dif- 
ferent, and we also know that an igneous dyke 
may traverse an alternation of more or less per- 
meable strata. Where the supply of water was 
greatest, conductivity and other things being 
equal, there would take place the greatest amount 
of diffusion of that liquid through the igneous 
magma. Were this latter a perfect fluid, and non- 
viscous, the more aquiferous, probably lighter, 
part would soon diffuse itself in all directions, 
rendering the whole a homogeneous mass. V e 
know, however, that all lavas are exceedingly 
viscous, especially the more highly silicious ones, 
and therefore such diffusion would take place very 
slowly. This would be aided by the upper part of 
the column being lighter, from that portion being 
placed under the more favourable conditions for 
absorbing water. 
The more porous the strata the greater the 
tendency will there be for the conduction away of 
the heat of the magma, either directly or by the aid 
of convexion currents of water, or by the conversion 
of the water into vapour, where pressure is so iow 
as to permit it. 
Extrusion or Eruption of the Ljneous Mapna 
into the Atmosphere . — If we suppose, simply for the 
sake of brevity of argument, the lava canal to be 
a tube of uniform size between the source of 
igneous matter and the surface (which, however, it 
never is), and that such a canal traversed rock strata 
of different permeability; then the magma enclosed 
in the tube would consist of a series of more or 
less saturated aquiferous strata, superposed on 
each other in the same order in which each part was 
exposed to a portion of the canal wall. Now, 
should a sudden exit of magma occur from the 
tube at its upper extremity, the expansion, or, in 
other words, eruption, would take place with a 
violence directly proportional to the amount -of 
dissolved water, and the temperature of that por~ 
