CALCAREOUS FORMATIONS OF THE SOLOMON GROUP. 559 



being apparently an " Orbicella " (Dana). The larger mass measured 4 feet 

 across ; and both were in the position of growth. These imbedded corals 

 were elevated about 40 feet above the sea, and from their position in the base 

 of the cliff they must have been buried beneath not less than 80 feet of coral 

 rock ; whilst below them, on which in fact they almost rested, was the friable 

 earthy deposit, or in other words, the partially consolidated ooze on which the 

 reef began to grow, or over which it extended by lateral expansion. A coat- 

 ing of calcareous tufa concealed from view the upper portion of the face of 

 the cliff, and thus prevented my examining its structure. It was evident 

 that the undermining action of water which found its way between the base of 

 the cliff and its soft foundation, assisted by the earthquake-shocks to which 

 this region is liable, had caused large masses of the cliff to become detached, 

 thus exposing the foundation of these upraised coral reefs. Another locality 

 in this island in which I was able to find the rock subjacent to the coral 

 limestone was on the western slope of the summit at an elevation of 200 feet 

 above the sea. Here I found a stream issuing from the line of junction of the 

 coral rock and an underlying earthy friable rock. On the south shore of Port- 

 Mary I found masses of reel ochre, protruding above the reef-flat. 



Before drawing any inferences as to the thickness of the coral limestone in 

 this island, I will refer briefly to the elevated eastern portion of the rim (vide 

 Plan and Section PI. CXLIV. figs. 5 and 6). Near the summit, and at an eleva- 

 tion of 400 feet above the sea, I found a volcanic rock (a hornblende trachyte) 

 exposed in situ in the bed of a rivulet. Small fragments of this and of one or 

 two other volcanic rocks (mostly gabbros) occur mingled with masses of coral 

 rock on the summit, and are frequent in the small basin-shapeel hollow there 

 situated. With these exceptions, coral limestone forms the surface of the most 

 elevated portion of the island, sometimes displaying faces 25 feet in thickness 

 even at the lip of the summit. The occurrence, however, of a rubbly conglomerate 

 of various volcanic rocks (for the most part gabbros), cemented together by a 

 calcareous material, at the foot of the eastern slope of the island affords sufficient 

 evidence of the proximity of the parent rocks, some of the blocks being from a 

 quarter to half a ton in weight. In accordance with the greater age of the 

 eastern side of the island as dry land, we should have expected to find it almost 

 stripped of its calcareous coverings. 



From the foregoing remarks, the following conclusions may be drawn as to 

 the history of the formation of this island. A submerged volcanic peak, 

 having been covered by the accumulation of a deposit resembling a deep-sea 

 mud, became the base of a coral-atoll, which has been subsequently upheaved 

 together with its foundations, to a height of nearly 500 feet above the sea. The 

 thickness of the coral crust probably does not exceed 150 feet, thus corre- 

 sponding with the limit of depth at which reef corals are supposed to thrive. 



