172 
THE GEOLOGIST. 
Castle Island is composed of gneiss, in which is found a mixture 
of a dark purplish grey felspar, fusible green hornblende and grey 
quartz, as observed by Capt. Campbell in 1827 ; the gneiss is capped 
by over-lying amorphous basalt fifty feet thick, nine hunilred and 
ninety feet long, and two hundred and ten feet wide in its broadest 
part, which is near the centre. This mass of basalt is supported by 
an aggregation of basaltic columns, of which some reach to the 
height of twenty-five feet. They possess the usual characters, are 
vertical, in close contact, varying in size and the number of their 
sides, and are jointed. Capt. Campbell determined their base to be 
a hundred and eighty feet, with their summits two hundred and fifty- 
five above the water. This is fifty feet more than is mentioned by 
Bayfield ; but, as the cohimnar and amorphous basalt have perpen- 
dicular sides, its thickness was made out by a plummet to be seventy- 
five feet, the feature of most importance in relation to the caverns. 
The summit is flat, and covered with moss and turf ; its shape is 
oblong, and the columns pass all around it, and thus explains their 
fortification-like appearance on entering Henley Harbour. The 
island itself resembles a fish in shape, with a liroad head, and having 
a distinct tail, which forms Chateau Point. It is a little over a mile 
and a quarter long, and a third of a mile broad at its northern part. 
(See map, plate vii.). 
Henley Island is situated to the north-east of Castle Island, from 
which it is separated by a narrow channel leading into Henley Har- 
bour, about a hundred and twenty yards wide, which is called by the 
fishermen Castle Reef Tickle. The shape of this island is that of a 
triangle, its most important side fronting towards the sea, and run- 
ning due north and south ; its southern side is hollowed out into two 
bays, which leaves the south-western part of the island in tlie form 
of a hill two hundred and four feet high, capped by the basalt as in 
Castle Island, and possessing all the characteristics peculiar to that 
island, with its pillars of the same substance. The extent of the 
basalt is about a fourth of that on the sister island, the width of this 
part of the island containing it being about two hundred and seventy- 
five yards. On that side only towards the sea (east) are the columns 
visible ; but as three caverns are there present, it was looked upon 
by Lieutenant Baddeley, R.E., as strong presumptive evidence that 
these basaltic columns traversed the mountain, a supposition which 
it appears to me to amount to a certainty, on compai'ing the two 
islands with one another. In these caverns (which must at one time 
have been Fingal's Caves in miniature) the columns possess the 
same regularity and juxta-position as they do on the outside. The 
largest was found by Captam Campbell to be twenty yards 
deep by fifteen yards in the middle ; the floors were strewn 
with the fragments of columns, and the sides were ornamented 
by those which their removal exposed to view ; the ceiling was 
as smooth as that of a room, but of almost an iron blackness. 
The thickness of the amorphous basalt above was estimated 
at fi-om thirty to forty feet, its com-se on both islands is from 
