o92 MR. J. J. LISTER ON THE GEOLOGY 



mound. The volcano was still in eruption the following year. In 

 1887 the height was estimated as 290 feet by a French man-of- 

 war ; and from a sketch taken in that year, it appears that the 

 summit of the island presented a rugged, uneven outline *. 



The * Egeria ' visited the island in Oct. 1889. In the four years " 

 that had elapsed since its formation, a large portion had been 

 removed by the action of the sea. Onlj' a part (perhaps one third) 

 of the original wide-based mound remained. This was limited for 

 half its circumference by a cliff which was highest in the middle 

 (153 feet), where it faced south, and subsided gradually at either 

 end. From the summit of the cliff the surface of the mound sloped 

 evenly downwards until it reached the level of a wide flat marked 

 by tidal ridges, which extended round that part of the circumference 

 of the mound not limited by the cliff. Landslips from the cliff 

 were of frequent occurrence at high water ; as many as twelve 

 were seen in one afternoon, and the ground round the edge of the 

 cliff was traversed by concentric cracks, showing where slices had 

 already become loosened preparator)^ to their descent. Capt. 

 Oldham erected a line of cairns at equal distances, extending from 

 the edge of the cliff — down the slope of the mound and over 

 the flat — as a means of measuring the rate at which the island is 

 reduced. 



The mound consisted of layers of finely-divided volcanic ash. 

 On the face of the cliff the layers were marked b}- the salts which 

 had crystallized at the surface more abundantly in some than in 

 others, and it might be seen that each layer was thickest at the 

 highest part of the mound, and gradually thinned out towards the 

 periphery. 



Numbers of volcanic bombs were scattered over the slope of the 

 mound, being largest and most numerous at the highest part. 

 These appear to have been ejected at the close of the eruption, as 

 none appeared in the cliff-section ; and the even, unscored outline 

 of the slope negatived the idea that they had accumulated at the 

 surface by the removal of any considerable quantities of finer 

 materials. 



The lavas presented all stages of vesicular structure, and some 

 of the bombs displayed a distinct spii'al twist in the surface 

 ridges. 



Mr. Harkert has examined the lavas and finds that they are 

 basic augite-andesites of specific gravity 2-708. 



The temperature of the interior of the mound was still high at 

 the time of our visit. In a hole 7 feet deep a thermometer rose to 

 100° Fahr., while it registered 77°-5 at the surface. 



To the south of the island there was an extensive shallow area 

 with 3 fathoms of water over it. It appears that this was, in part 

 at least, occupied by that portion of the original mound which has 



* The above details are taken from an account of the island by Capt. 

 Wharton, E.N., F.E.S., published in 'Nature' for Jan. 23, 1890 (pp. 276- 

 278). This account is accompanied by figures and a map. 



t Geol. Mag. for June, 1891, p. 250. 



