84 Revieivs — Oliver's Geology of St. Helena. 



the rocks of the two are different. Whether the place of an old 

 outburst, or not, the " Bam" consists of enormously tilted lavas, and 

 requires further examination. 4. Capt. Oliver thinks that the 

 coloured clay beds, in the Longwood District, are due to the surface 

 decomposition of thick, hard claystones, and not of scoriae. 5. He 

 points to the existence of a great submarine terrace around the 

 island, sloping outwards to a depth of 50, 70, 100 fathoms and more, 

 and averaging nearly two miles in width, with an abrupt descent 

 into very deep water. The prolongation of the plane of the 

 existing land approximately touches the outer margin of this teiTace, 

 quickly if the land surface slopes rapidly, and at a lower angle if 

 the land slopes gently (according to the dip of the strata generally) ; 

 in fact, according to the greater or less inclination of one slope, so is 

 that of the other ; the submarine terrace, however, is less inclined 

 than the slope of the land. Hereby some of the physical changes of 

 this old island are clearly illustrated. Doubtless the land reached a 

 much greater elevation — some thousands of feet — with a wider base 

 at some far distant period, when the accumulation of the volcanic 

 matter, and the uphfting of the mass, formed a lofty conical island, 

 with sloping coasts. Into these in time the sea ate an enormous 

 terrace-like notch, or littoral groove ; and after a subsidence, it 

 began to eat away the present coast, and to grind down and disperse 

 as mud "the enormous cubic mass of hard rock which has been pared 

 off the circumference of the island," a denudation so great, says 

 Darwin, that " the mind recoils from an attempt to grasp the 

 number of centuries of exposure" necessary for its production ; and 

 yet this coast-notch is neither the first nor the greatest that has 

 •margined St. Helena. 



Whether or no these were the only great changes of level, some 

 sloping shore must have existed, in place of the precipices of to-day, 

 for the upblowing of the calcareous shore- sand, on to the sides of 

 valleys and flanks of hills six hundred feet above the sea, and 

 forming the superficial limestone, well described and accounted for 

 by Darwin. Many movements of oscillation, besides those of local 

 disturbance, may have occurred ; and the enormous denudation the 

 surface of the island has suffered may be due in part to sea-action ; 

 but in a far great degree it is attributable to rainfall and snow-water, 

 due to the exceeding loftiness, or to some old "pluvial period." 

 The amount of moisture at present is sufficient to give luxuriant 

 vegetation to the central heights, and brooks in abundance ; but it 

 effects a very trifling amount of superficial change. 



To resume : — On many other points besides the origin of the 

 superficial calcareous deposits, and the enormous denudation, our 

 authors fully agree — 1. The volcanic origin of the whole mass of 

 the Island. 2. The existence of the " basal series," fi'om the Barn 

 to Prosperous Bay, much intersected by dykes, upturned, and over- 

 lain by newer beds of lava. 3. The distinct character of the Flag- 

 staff Hill and Barn. 4. The extent and characters of the " Basaltic 

 scries," doubtfully of submarine origin. 5. The great crater, at 

 Sandy Bay, as the origin of most, if not all, of the eruptions. The 



