172 



THE GEOLOGIST. 



Castle Island is composed of gneiss, in wliicli is found a mixture 

 of a dark pm-plish grey felspar, fusible gTeen hornblende and grey 

 quartz, as observed by Capt. Campbell in 18-27 ; the gneiss is capped 

 bv over-lying amorplions basalt fifty feet tliick. nine hundi'ed and 

 ninety' feet long, and tvro liundi-ed and ten feet wide in its broadest 

 pai-t, Vliich is ueai- the centre. This mass of basalt is supported by 

 an aggi'egation of basaltic columns, of which some reach to the 

 height of twenty-five feet. They possess the usual chai-acters, are 

 vertical, in close contact, varying in size and the number of their 

 sides, and are jointed. Capt. Campbell determined theii' base to be 

 a hundi'ed and eighty feet, with their summits two hundred and fifty- 

 five above the water. This is fifty feet more than is mentioned by 

 Bavfield ; but. as the columnar and amorphous basalt have perpen- 

 dicular sides, iis thickness was made out by a plummet to be seventy- 

 five feet, the featui'e 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 Harbom*. The 

 island itself resembles a fish in shape, with a broad head, and having 

 a distinct tail, which forms Chateau Point. It is a little over a mile 

 and a quarter long, and a thii'd 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 nai'row channel leading into Henley Har- 

 bour, about a hundi'ed and tvrenty yai'ds "wide, vdiich is called by the 

 fishermen Castle Eeef 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 the form 

 of a hill two hundi'ed 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 fom'th 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. K.E., as strong presumptive evidence that 

 these basaltic columns traversed the mountain, a supposition which 

 it appears to me to amoimt to a certainty, on comparing the two 

 islands "v^uth one another. In these caverns (which must at one time 

 have been FingaTs Caves in miniatm-e) the columns possess the 

 same regularity and juxta-position as they do on the outside. The 

 largest was found by Captain 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 theii' removal exposed to ^uew : 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 from lliirty lo forty feet, its coiu'sc on both islands is from 



