THE MED IT E 1 ‘ KANE AN NATURALIST 
375 
w 
Great caution is needed in determining where 
the lower horizon of the Clays is situated, as no 
little experience is necessary to be able to distin- 
guish between the transition bed of the Marls and 
Clays and the bands of argillaceous blue lime- 
stone which are distributed at various levels 
throughout the Globigerina- rock. 
The most accurate measurements are those that 
were obtained in the well-shafts, and in the sec- 
tions that were cut by the Engineer Corps at Gebel 
Imtarfa during the construction of the new road. 
The extreme thinning-out that the above measure- 
ments indicate is in every case due to local depres- 
sions. If |a line be drawn from Gebel Ciantar 
to Selmone it will pass through the localities in 
which the Clays are most attenuated. The at- 
tenuation is apparently due to the north-east-by- 
east dip, which the strata have from the Great 
Fault to the eastern extremity of Malta. The 
pressure of the superincumbent strata on the 
Clays is therefore the greatest in that direction, 
with the result that the Clays have been compres- 
sed and thinned out at the outcrops on the east 
and north-east. In the shafts that have been cut 
in the Boschetto and Gomerino Valleys the Clays 
just exceed 20 feet in thickness, whereas at the 
eastern extremities of these valleys and of others 
situated along the same line the thickness 
barely exceeds 10 feet The strata lying bet- 
ween the northern side of the Great Fault and 
the fault which bounds the southern side of St, 
Paul’s Bay dip in an easterly direction, so that the 
thickest part of the Clays is found on the western 
side at Ghain Tofliha and Karra ba, and the most 
attenuated along the western boundary of the 
Nasciar Plain. 
In Gozo the thickness of the formation is not 
so variable, owing to the comparative absence of 
faults and the more uniform horizontally of the 
strata. At Fort Chambray, and along the shores 
of the bay at its foot, the Clay outcrop varies from 
30 to 40 feet in thickness; but the taluses that 
have been formed along the slopes make the for- 
mation appear to be at least 120 feet thick. At a 
distance of J mile east of Chambray a vertical 
section shows the Clays to be only 10 feet thick; 
this thinning-out is also attributable to the marked 
southerly dip of the overlying strata. In the 
isolated hills and smaller plateaux this dependence 
of the thickness of the Clay-formation upon the 
dip of the beds that overlie it is still more 
strikingly shown. 
■i-r 
At Ghain Tofliha, in Malta, a huge mass of rock 
having an area of about 100 acres, has been 
detached from the cliffs, and has fallen so as to 
dip towards the cliff of which it was formerly a 
part. The Clay-bed has been dislocated, and 
w hile on the shore side it shows a thickness of 20 
feet, at the point of dislocation the bed has been 
thinned out to a few inches. 
But is not only along the outcrops that these 
differences in thickness occur. The formation is 
overlain conformably by the Greensands, the line 
of demarcation between the two being as a rule 
well defined; but in some localities the transition 
is shown, by the admixture of the Clay and Green- 
sand, to be of a very gradual character. This line of 
separation is by no means uniformly horizontal, and 
in places is takes an undulatory form. Both the 
Clays and the Greensands, therefore, vary greatly in 
their thickness, and it often happens, as at Diugli, 
where the Greensands are 50 feet and the Clays 
only 6 or 10 feet thick, that the maximum deve- 
lopment of the Greensands is accompanied by a 
minimum thickness of the Clays. This undulatory 
surface of the Clays forms a series of natural 
reservoirs in which the fains of winter are stored, 
and it is from these that the population of the 
Islands derive their water-supply. 
III. Lithological axd Mineealogical 
Chaeactees of the Steata. 
A microscopic examination of numerous speci- 
mens that had been taken from various horizons (1) 
of the Clay-formation revealed the general struc- 
ture of the rock, and sho*wed- it to consist of tests 
of foraminifera and minute fragments of minerals, 
the most numerous of which were oxide of iron 
and glauconite. Rounded grains of quartz, augite, 
hornblende, felspars, zirc.on, and tourmaline (2) 
were also present in comparative abundance in 
every part of the Clay, but more especially so in 
( 1) Besides the many Bides that I prepared my- 
self , I was enabled, through the courtesy of Dr. 
John Murray , to examine the numerous sections 
winch hr hod caused to be prepared, and which he 
subsequently described. 
(-) Dr. John Mum ay, 'The Maltese Islands etc., 
< Scot. Geogr. Mag. voi. VI (1890 ) p. J/fj. 
