THE KOYAL ARTILLERY INSTITUTION. 
295 
ON THE 
GEOLOGY OE GIBRALTAR 
WITH ESPECIAL REFERENCE TO THE RECENTLY EXPLORED CAVES AND 
BONE BRECCIA. 
BY Lt. ALEXANDER B. BROWN, R.A., F.R.A.S., E.G.S. 
The principal mass of the Rock appears to be a grey dense primary 
marble, the beds of strata varying from 20 to more than 40 ft. 
° f . the in thickness, with subordinate beds of shale, and chert (speci- 
mwbie! imary mens No. 13* from Windmill Hill, and No. 9 from above that 
place) with rubbly sandstone dipping in a direction from 
E. to W. at an angle of about 37°; there are several casts of marine shells, 
chiefly terebratulse, found in it belonging to the oolitic period; the Rock 
extends 2^ miles from N. to S. and is about J miles in breadth, the greatest 
elevation being about 1470 ft. The soil on which the town is built, the 
Alameda, &c., is siliceous red sand, this forms the largest bank 
raUand ° f arenaceous matter on the western side of the mountain, and 
consists of several particles of crystallized quartz, nearly 
colourless in themselves but of an ochreous or red colour in mass, by 
reason of the red argillaceous earth which adheres to them. The sand bank 
on the eastern side of the mountain is composed of small 
Cai_ of particles of the calcareous rock, the whole of a whitish grey 
eastern beach, colour, there also appear to be some raised beaches of siliceous 
deposit partly broken by the fall of the crumbly calcareous 
rock, &c. On the south of the red sand the soil is variegated, it is a light 
loose mould of a fertile quality, and is found irregularly scattered in the 
fissures, and crannies of the rock, which in the wet season becomes of a 
saponaceous sliminess* In other places the mould is mixed with sand, and in 
„ .. „ . . other spots the soil consists of a stiff marl or species of fullers*- 
earth, and may be looked upon as alluvial. 
There cannot be a doubt that at one time the Rock was the bed of agitated 
waters, and only arrived at its present position by successive upheavals, 
manifested by the numerous pot holes found in different parts. On 
Windmill Hill which was doubtless a sea-beach at one time, 
Pbt boles. J have found some of these pot holes filled with coloured 
* The Nos. refer to those on specimens in the R.A. Institution Museum. 
[VOL. V.] 
40 
