570 H. B. GUPPY ON THE RECENT 



lower slopes. Such is without doubt the geological structure of Cape 

 Keibeck, which has an elevation of over 1100 feet at the coast. I traced the 

 coral limestone as high as 500 feet above the sea when examining this cape, 

 this being the greatest elevation at which the coral rock came under my 

 observation in St Christoval. In many parts of the coast a fawn-coloured 

 crystalline limestone, with homogeneous texture, in which sometimes reef 

 debris can be observed, in fact just such a rock as might arise from the 

 consolidation of the white ooze of coral reefs, takes the place of the coral rock. 

 I followed it up to about 500 feet above the sea. It is probably of no great 

 thickness, its relation to the underlying volcanic rocks being often displayed in 

 the courses of the smaller streams. 



I will here relate my experience in the case of a hill that lies on the east 

 side of Wano Bay, since it is very typical of my experiences in other parts of 

 this coast. During two different ascents, in which I purposely avoided the 

 courses of the streams, I traced the coral rock on the surface to a height of 150 

 feet above the sea, and at the summit, 400 feet up, I found the fawn-coloured 

 limestone before alluded to. In no place during these ascents did I find a 

 fragment of volcanic rock at the surface ; but when, taking advantage of the 

 deep gulleys that had been worn by the streams into the mass of the hill, I 

 availed myself of the rude sections of the hill's substance there displayed, I 

 found in position the old volcanic rocks (altered dolerites). 



These volcanic rocks are not uncommonly exposed protruding in mass above 

 the flats of coral formation which skirt the coast. Doleritic rocks project 

 through the coral rock on the point east of Wano Bay ; whilst in the vicinity 

 of Cape Keibeck, a dolerite, from which the coral rock that originally invested 

 it has been but partially removed, protrudes in mass to a height of 10 or 12 feet 

 above the reef-flat. Pebbles and small blocks of volcanic rocks are frequently 

 to be observed in the reef-rock all along this north coast of St Christoval. 



With reference to the estimation of the thickness of these encrusting 

 calcareous deposits, I have no certain data. The fawn-coloured limestone, as 

 has been remarked above, can be of no great thickness ; whilst the coral 

 limestone, which I rarely saw exposed in faces exceeding 20 feet in height, 

 probably constitutes a thinner crust than I have found in other parts of the 

 group. Without doubt these calcareous envelopes originally extended to far 

 higher levels, from which they have been stripped by the agency of subaerial 

 denudation, which in these regions of great rainfall must be very rapid. From 

 my observations of the rainfall in this archipelago, I am inclined to estimate 

 the annual amount at usually not much under 150 inches. 15 ' Of the rapid 



* This estimate refers only to the coast. Tn the elevated interiors of the larger islands the annual 

 f ill is probably twice as much ; whilst on the lofty summits of Bongainville and Guadalcanar the rain- 

 faJl is still greater. 



