chap. iv. Flagstaff Hill and the Barn. &j 



rocks. These two lofty hills are connected (as is shown 

 in the accompanying woodcut) by a sharp ridge, 

 which is composed of the rubbly lavas of the basal 

 series. The strata of this ridge dip westward, the in- 

 clination becoming less and less towards the Flagstaff; 

 and the upper feldspathic strata of this hill can be seen, 

 though with some difficulty, to dip conformably to the 

 WSW. Close to the Barn, the strata of the ridge are 

 nearly vertical, but are much obscured by innumerable 

 dikes ; under this hill, they probably change from being 

 vertical into being inclined into an opposite direction ; 

 for the upper or basaltic strata, which are about 800 

 or 1,000 feet in thickness, are inclined north-eastward, 

 at an angle between thirty and forty degrees. 



This ridge, and likewise the Barn and Flagstaff Hills, 

 are interlaced by dikes, many of which preserve a re- 

 markable parallelism in a NNW. and SSE. direction. 

 The dikes chiefly consist of a rock, porphyritic with 

 large crystals of augite ; others are formed of a fine- 

 grained and brown-coloured trap. Most of these dikes 

 are coated by a glossy layer, 1 from one to two-tenths of 

 an inch in thickness, which, unlike true pitchstone, 

 fuses into a black enamel ; this layer is evidently ana- 

 logous to the glossy superficial coating of many lava 

 streams. The dikes can often be followed for great 

 lengths both horizontally and vertically, and they seem 

 to preserve a nearly uniform thickness : 2 Mr. Seale 



1 This circumstance has been observed (Lyell, ' Principles of 

 Geology,' vol. iv. chap. x. p. 9) in the dikes of the Atrio del Cavallo, 

 but apparently it is not of very common occurrence. Sir G. Mac- 

 kenzie, however, states (' Travels in Iceland,' p. 372) that all the 

 veins in Iceland have a 'black vitreous coating on their sides.' Capt. 

 Carmichael, speaking of the dikes in Tristan D'Acunha. a volcanic 

 island in the southern Atlantic, says (' Linnasan Transactions,' vol. 

 xii. p. 485) that their sides, ' where they come in contact with the 

 rocks, are invariably in a semi-vitrified state.' 



2 * Geognosy of the Island of St. Helena,' plate 5. 



