THE MEDITERRANEAN NATURALIST 
25 ? 
from the island’s formations at a time when the 
forces of denudation were much more active than 
they are at present. The numerous oscillations of 
level that the islands bear evidences of having 
undergone, and the extensive erosion to which 
they were afterwards subjected by the combined 
action of frost and rain, are probably to be ac- 
counted among the most potent of the forces that 
assisted in planing down the islands’ rocks, and in 
distributing the debris throughout the Quaternary 
and the Recent deposits. The thin integument of 
soil that covers the surface even in those localities 
where denudation has been the most severe, 
masks from view the rocks beneath; and this, 
combined with the fact that but few clean cut 
sections of the Lower Coralline Limestone are 
exposed inland, affords an explanation for the 
obscurity of these thin veins of black limestone, 
and for the doubt with which their occurrence in 
the Malta rocks has hitherto been regarded. 
Geological Magazine. 
Fighting Mice with a Bacillus. 
Professor Loeffler’s crusade against the field 
mice of the Thessalian plain has enden in victory. 
The latest reports announce that the fields are 
strewn with the corpses of mice. It will be re- 
membered that Professor Loeffler discovered some 
time ago a new bacillus, the “ bacillus typhi mu- 
rium, ” which has the power of producing a certain 
disease in ndce, and in mice alone. A plague of 
field mice, threatening to destroy the harvest, 
having appeared in Thessaly, he was appealed to 
by the Greek government, and immediately started 
for Athens. He began his experiments by treating 
field mice in the laboratory with injections of his 
bacillus cultivation, and when these experiments 
showed his method to be undoubtedly the right 
one, he started for Thessaly with a staff of Greek 
doctors. Bread crumbs, saturated with the bacilla- 
ry substance, were strewn broadcast over certain 
fields, and as early as a week later the results were 
visible. Success being now assured, Professor 
Loeffler will return to Germany, and the bacillus 
cultivation will be carried on at the seat of war 
itself. Scientific American. 
Teeth Food. 
In an address on “tooth culture” at a meeting of 
British dentists Sir James Crichton-Brown referred 
to the withdrawal of fluorine from our food as one 
of the important causes of the increase of dental 
caries. This element seems to find its way into 
the animal economy only through the silicious 
steams of grasses and the outer husks of grain, in 
which it exists in comparative abundance; and 
the fineness and whiteness now demanded in flour 
are secured by removing the bran that yielded 
fluorine to our ancestors. The enamel of teeth has 
been found to contain more fluorine, in the form 
of fluoride of calcium, than any other part of the 
body. Fluorine might, indeed, be regarded as the 
characteristic chemical constituent of this struc- 
ture, which is the hardest of all animal tissue, and 
contains 95. 5 per cent of salts against 72 per cent 
in the dentine; and any deficiency of this element 
must result in thin and inferior enamel. The 
question of restoring the supply of fluorine to 
our diet is worthy of consideration. 
The Relationship of the Structure of 
Rocks to the conditions of their 
Formation. 
By H. J. Johnston La vis, m.d.,f.g.s,, 
The Presence of Volatile Matter in Modifying 
the Structure and Composition of Igneous Rods . — 
As has already been intimated, those grand explo- 
sive eruptions that burst forth after long intervals 
of complete tranquillity are characterized by an 
essential ejectamenta of vesicular structure and 
fragmentary state. On the other hand, chronic 
activity, even when it increases to the stage of 
paroxysmal outbursts, is equally well marked by 
the outflow of a continuous mass of igneous magma, 
or what is generally known as lava. The vesicu- 
lar rock masses, or scoria, that cover lava streams 
are, both in origin and structure, widely, though 
not completely, different from the pumiceous pro- 
ducts of the first kind of eruption. These assertions 
hold true almost without exception in the case of 
basic rocks, and are exceedingly common even 
amongst the most acid ones. Most of the illustra- 
tion. that will be brought forward have been 
chosen from among basic rocks, since, so to speak, 
crisis between a vitreous, fine, vesicular, and 
fragmentary state on the other, is easily attained, 
and is well defined. 
