194 
THE MEDITERRANEAN NATURALIST 
salbands of sone dykes, being smaller than those 
in the more central part. 
On the other hand, we may meet with various 
intrusive rocks with more or less purely vitreous 
salbands, in which, in many cases, the line cf 
demarcation is often distinct and very sharp 
between the dyke walls and the intrusive matter. 
There are also cases, as in the dykes of liparite of 
the Ponza isles, which possess thick pitchstone 
salbands which are soldered to the walls of the 
quartzose tufas. It would seem that the great 
resemblance of the two rocks in chemical and 
mineralogical composition, and therefore the small 
difference between their points of fusibility, a 
very slight excess of temperature in the intrusive 
rock would be sufficient to fuse the walls, and yet 
cool rapidly enough to prevent complete crystal- 
lization, thus leaving the vitreous salband. This 
is aided, no doubt, by the low heat conductivity 
of the surrounding tuffs. Even in granite, al- 
though there is no vitreous salband which would 
be incompatible with its coarse, well-crystallized 
structure, Naumann describes granite dykes in 
which the grain is finer towards the margin. 
It seems probable that where intrusion takes 
place into rocks, the cleavage planes of which are 
nearer the horizontal in direction, the loss of heat 
will take place slowly, and we. should expect to 
find coarse-grained granites and trap rocks; 
whereas, the more the cleavage planes approach 
the vertical the greater will be the rapidity of 
cooling. This is a question well worth inquiring 
into, for Jannettaz (1) has show that the major 
axis of the isothermic ellipsoid in crystals is 
parallel to the principal planes of cleavage, and in 
rocks with the planes of schistosity. (2) 
( to be continued ) 
( 1 ) Memoires sur Id propagat-ton dc la ckaleur 
dans lea corps crista! Uses (Ann. Ch. et Ph., Jf 
serie, t. XXJ.X.,p. 5; Pnd l dc I a societe Geologique 
de France , 3° serie , t. P r et suiv.) 
(2) As the stratification of strata in general ap 
proaches nearer to tin horizontal than the vertical, 
the. conditions will he most favourable for the 
retention of the int< rnal heat of our globe. 
Subterranean Waters in the Sahara. 
The admirable results which have attended Tie 
artesian borings in the "Wed Piir. at V.'argla, and 
more recently at El Golea in the Sahara, have led 
to a demand being made by the inhabitants of the 
Mzab in the southern part of the French Sahara, 
for the assistance of the Government in under- 
taking experimental borings in that region also. 
M. G. Holland, one of the few geologists who have 
explored the Algerian Sahara, and the only one 
who has visited the extreme south, makes the 
following observations on the regime of subterra- 
nean waters between Laghuat and El Golea. 
From the north to south in the region of the Laya, 
and on the chalk plateau which extends to the 
south, borings have no chance of success. In the 
shebka of the Mzab and of Metlili, the conditions 
are only moderately favourable, and it would be 
necessary to penetrate down t TOO and even to 
1000 feet. To the south of the 32nd parallel the 
chances of success increase in what M. Holland 
calls the shebka of the south of El Hassi. Borings 
would undoubtedly succeed in the depressions of 
Dayet Tarfa, El Aref, Zubia, and Bn Fakrun. 
Further south, springing water would be obtained 
along the western border of the chalk reliefs^ 
which is unfortunately complicated by the ramifi- 
cations of the Western Erg, and the depths of the 
borings would go on decreasing until, on approa- 
ching the region of El Golea, it would be necessary 
to penetrate down only to 400 feet. 
The Malta Potato Disease. 
Productive as is the soil of the Maltese Islands 
the advantages that it offers to its cuitiv.t. rs are 
often more than counter-balanced by the destruc- 
tive attacks of the numerous animal and vegetable 
parasitical pests with which the produce is from 
time to time assailed. 
As an example of the uncertainty attendant 
on the efforts of the Maltese agriculturist, wo may 
state that in the. early part of the Spring of the 
present year, the potato crop was especially abun- 
dant, aud husbandman and merchant alike looked 
forward to some compensation for the numerous 
