48 
THE MEDITERRANEAN NATURALIST 
paper to the Royal Society, in which he estimates 
that the level of no strain is now, after the lapse 
of about 174 million years, about five miles deep, 
while Darwin places it at two miles deep in 100 
million years since consolidation, and the Rev. 0. 
Eisher at less. My own calculations agree pretty 
closely with the two latter, but the depth will vary 
according to the assumed data of consolidation, 
co-efficient of expansion of rock, Ac., upon which 
the estimate is based. The agreement of these 
independent investigators is, however, remark- 
able. 
Theoretically, the depth of the level-of- no-strain 
varies as the time, and the amount of rock crushed 
may be represented by a cone the base of which 
is the difference in area between the surface of the 
globe before, and the surface after contraction 
having a height equal to the depth of the level-of- 
no-strain. It will thus be seen that the cubic 
quantity of rock crushed will be only one-third of 
what it would he with a crust the thickness of the 
depth of the level-of-no-strain and unaffected by 
cooling in its own substance. It follows from 
these various considerations that the compression 
of so thin a shell is inadequate to account for the 
upheaval of mountain ranges as we see them upon 
the earth, and that geological facts are eloquent 
against this explanation of what is colloquially 
called “mountain building.” 
The discovery of the existence of a neutral zone 
or level-of-no-strain in a cooling globe has revo- 
lutionised geological thought as is clearly stated 
in the able Smithsonian review of Geological 
Science for the years 1887 and 1888 by Professor 
Me Gee. 
An examination of most great ranges show that 
they have a central core of gueissic and granitic 
rocks forced up through the overlying sedimen- 
taries, which are folded into loops between the 
intrusive tongues of gneiss. 
The phenomena of mountain structure lead us 
to the conviction that the lateral pressure increases 
with the depth instead of diminishing, as it should 
do on the contraction hypothesis. 
It is from these considerations, together with a 
wide range of geological inferences, that lead me 
to reject the contraction explanation of mountain 
formation, and to seek for another more in con- 
formity with nature Ip a future number T pro 
pose to give an outline of my own views as to the 
structure and origin of these great corrugations 
of our earth. ( To he continued.) 
Observations on the Geology of the 
Maltese Islands. 
by John H. Cooke 
( continued .) 
The caves & fissures of the Islands : — 
Like all freestone districts, the Maltese strata 
present unequivocal evidences of the destructive 
forces of the elements air and water. In the course 
of his peregrinations around the cliffs and through 
the gorges of the islands, the attention of the 
observer will be especially attracted to the great 
number of caverns that occur in their limited area. 
They are remarkable, however, for their number, 
rather than for their proportions, for few of them 
attain any considerable size. This is largely due 
to the nature of the strata in which they have been 
formed, as, owing to their comparative softness the 
caverns usually collapse when they reach a certain 
limit. But small as they are, most of them are 
invested either with a mythical or an historical 
importance. The ancient inhabitants of Malta, 
like those of the neighbouring countries, were ever 
ready to assign to the heroic all that w T as in the 
least incomprehensible to them. 
The folk-lore of nations teems with examples of 
the manner in which natural phenomena have 
awakened the curiosity and fear of the untutored 
mind, and it has been, when labouring under the 
delusions that such feelings as these excite, that 
the extravagances and fables, that are often 
handed down to us as history, have had their 
origin. 
ISTuma is said to have consulted Egeria in a cave, 
which is still shown at Rome; and the caves of 
| Sicily have, from time immemorial, been credited 
i as being the favourite haunt of a gigantic race of 
| ghouls. • 
In Malta, the grotto dedicated to Calypso, at once 
recalls the fable of Homer; while the Har-Hasan 
cave conjures up vivid lieroical scenes of the deeds 
of derring do that were formerly perpetrated on 
| the high seas by the chieftan with whose name 
; the cave is now associated, 
