Roclc Gibraltar^ and ac^aceni Country. 77 
peared to be contained in the older solid limestone, and to run. 
in strata conformable to it. 
Ample opportunities are afforded to gratify every wish of the 
geologist, in ascertaining the structure of the hill, by visiting the 
extensive excavations in various parts of the rock, which have 
been formed for the purpose of strengthening this great and im- 
portant fortress. At the foot of the hill, the sole rock visible is 
the conglomerate limestone, which occurs in great abundance, 
and forming small hills. The imbedded masses are often of a 
very large size. The basis is a red, coarse, calcareous cement, 
or a calcareous tuff, more or less hard, and often intermixed 
with round concretions of calcareous sinter. At the foot of the 
hill the rock is often almost entirely composed of this calcareous 
tuff. As we ascend the hill, this conglomerate rock decreases 
in quantity, the imbedded masses become smaller, and the con- 
necting basis less abundant, more compact, finer, and of a light- 
er colour. The imbedded masses, which are of every shape, 
are undoubtedly broken portions of the solid limestone nucleus. , 
When we have ascended above two-thirds of the hill, this con- 
glomerate encrusts the interior mass to the depth only of a few 
inches, and a little higher up almost entirely disappeai’s, when 
the solid limestone forms the whole upper part of the hill. 
That such is the structure of Gibraltar Rock, a central mass 
of old and solid limestone, covered to various depths by a new- 
ly formed conglomerate, such as has been described, appears, 
from the examination of those parts of the hill through which 
roads have been cut in the rock, of those long arches cut 
through both the conglomerate and solid limestone, and in par- 
ticular of those amazing excavations, as they are called, planted 
with cannon,' often running to a great extent, and parallel to the 
exterior surface of the hill, from which they extend into the 
rock from SO to 50 feet, cutting in various places through the 
conglomerate into the solid mass. Partly owing to the darkness 
in these long arches, and from other circumstances, I seldom 
could discover any well defined line of separation between the 
solid and conglomerate limestones, though such a separation 
certainly exists. The imbedded masses of the conglomerate, 
however, the nearer they approached the solid rock, lay closer 
