The Geology of Ascension and St. Helena Islands. 153 



directions, but the total result was an elongation of the island in 

 the sense of the dominant system of fissures. Nevertheless, the fl.ows 

 visible in nearly two-thirds of the island emanated from the fissures 

 concentrated in the Sandy Bay district. This region, before erosion 

 had far advanced, was a low-angled, exogenous, basaltic dome, the 

 flows of which dipped seaward on all radii, at an average angle of 

 eight or ten degrees. At the headwaters of Jamestown and Rupert's 

 valleys the Sandy Bay flows are seen to cover and wrap around the 

 steeper flows from the older Knotty Ridge-Deadwood centre. 



A third concentration of fissures feeding lava-flows was located 

 under the sea, just off the shore of Manatee Bay. Flows and dikes 

 originating in that area are to be seen in the majestic Man-and- 

 Horse cliffs. 



Only one sill was demonstrated in the island. This is exposed in the 

 valley-wall near Wild Cattle Pound and close to High Hill. It is a 

 columnar lens nearly 40 feet thick in the middle and about 500 yards 

 long on the outcrop. It transgresses a few of the invaded flows of 

 basalt. 



Some of the St. Helena fissures have been locally enlarged into 

 " necks '" by explosion or by the melting action of blowpiping 

 gases. Monolithic bodies of rock; resisting erosion better than the 

 lavas and tuffs surrounding them, represent these exceptional, 

 truly " central " vents. In some instances remnants of the corre- 

 sponding steep cones of tuff and centrifugal flows vvere mapped. This 

 is true even at High Knoll and High Hill, where the necks are, in 

 ground-plan, elongated in the directions of the locally dominant 

 dikes, so that the hypothesis of local widening of the fissures is 

 rendered very probable. The largest neck is at Sliding Stones Hill 

 in the Sandy Bay district. There the widening seems to have 

 affected at their intersection two master dikes running nearly at 

 right angles to each other. The High Hill neck is similarly shaped 

 and similarly interpreted. In each case, enlargement of the fissures 

 through the melting of the country-rock is specially suggested by the 

 ground-plan of the neck. More nearly cylindrical necks were mapped: 

 at the hill crowned by the Ba])tist Chapel, Sandy Bay ; at the superb, 

 weU-known pyramid called '' Lot " ; at Castle Rock (Man-of-War 

 Roost) ; Hooper's Rock ; Sheep Knoll ; and at two bold crags in 

 Powell valley, respectively east and south-east of White Hill. 



Some of these '" necks " may be the monolithic fillings of craters, 

 such as would be formed if the Kilauea lava-lake of Hawaii were to 

 solidify during a high stand of the lake-level. Such a body would 

 be a " head " rather than a '" neck ", the " neck " being below and 

 invisible. 



Scenically imposing as the " necks " or crater-fillings are, the 

 output of the flows from these centres was probably very small, 

 when compared with the eruptions from the normal, narrow fissures. 

 Though the " necks " are most numerous in the Sandy Bay amphi- 

 theatre, none of them appears to have long functioned as a centre 



