154 Reginald A. Daly — 



of extrusion. The building of the great, composite, exogenous 

 Sandy Bay dome was rather due essentially to eruptions from many 

 fissures, which were opened in special abundance over an area 

 measuring at least 3 miles by 2 miles. In that sense Sandy Bay 

 was the chief " centre " of eruption in St. Helena. 



In relation to that area of concentrated fissaring, the vents at 

 High Hill and High Knoll have the positions of " subsidiary " or 

 " parasitic " cones, but both of these local vents are likely to have 

 been occasioned by fissuring which was independent of that 

 registered in the remarkable dike-system of the Sandy Bay district. 

 Another outlying vent formed an unusually high cone of tufi, with 

 some basaltic flows, on the ridge north of Swanley Valley, around 

 " The Saddle ". Several local tuff-cones, which had been deeply 

 buried under lava-floods, are exjDosed in sea-cliffs and canyon walls. 



Tv/o local vents, at Little Stone Top and Great Stone Top, are 

 of special importance, since their effluent lavas are alkaline rocks, 

 very like the Ascension trachytes. For preliminary descriptions 

 these rocks may be called trachyte, though laboratory study may 

 show the need of a different designation. At Little Stone Top the 

 trachytic magma seems to have risen in the form of a dome, greatly 

 broadened at the orifice and finally overflowing. One flow, 300 feet 

 thick, is now represented in remnant by Ben Cool en ridge ; another 

 outflow constitutes Boxwood Ridge ; a third crept down toward 

 the base of Great Stone Top. 



The vent by which the trachyte of Great Stone Top came up is 

 exposed on the shore near Elephant Rock. It is a dike-like pipe, 

 apparently about 300 feet long and between 30 and 60 feet in width. 

 One of its chilled contacts with the older flows of basalts is admirably 

 exposed. The contact with the gently dipping basalts is here nearly 

 vertical. Thence the trachytic magma rose along a dike fissure 

 obliquely, so that the upper part of the injection dips 60 degrees 

 to the eastward. Near the 800-foot contour the magma reached the 

 surface and spread out as a dome. The cap of this mushroom- 

 shaped eruptive is now 900 feet thick, after losing some of the 

 original thickness by prolonged erosion. 



In the cliffs due south of Little Stone Top, the older basaltic 

 flows, dipping about 10 degrees to the eastward, have been bent 

 up anticlinally by a thick wedge (or chonolith ?) of trachyte, which 

 is purely intrusive. The apex of the wedge is visible in the sea- 

 cliff at the 500- foot contour. The force of the injection sufficed to 

 steepen the dip of the invaded flows on the eastern side of the wedge 

 to 20 or 25 degrees, and on the west to change the dip to 4.5 degrees 

 to. the westward. Alongside is a 60-foot, inclined dike of trachyte, 

 conspicuous in the sea-cliff. A long, 15-foot dike of trachyte cuts 

 the basalts a little to the south-east of Ben Coolen ridge, and ranging 

 with it, towards Little Stone Top. This completes the list of known 

 occurrences of highly alkaline bodies in St. Helena, but possibly 

 some of the necks of the Sandy Bay district will prove to be allied 

 types of rock. 



