404 Dr. Jack on the Geology and Topography 



pressure of the superincumbent soil^ and the constant infiltration of water 

 through it. The species are obviously the same with those which now abound 

 under the neighbouring sea, and sometimes the transition from the recent to 

 the fossil coral is only effected by the gradual rise of the land from the shore. 

 Large Kima shells (Chama Gigas) are also found on the hills, exactly as they 

 occur on the present reefs, and are collected by the inhabitants for the purpose 

 of cutting into rings for the arms and wrists. Every thing seems to indicate 

 that the surface of the island must at one time have been the bed of the ocean_, 

 and that, by whatever means it attained its present elevation, the transition 

 must have been effected with little violence or disturbance to the marine pro- 

 ductions at the surface. The subjacent rocks are stratified ; among them I 

 found granular quartz, limestone, and calcareous sandstone*; towards the 

 southern end I also met with several kinds of limestone, some a coarse yellowish 

 white stone, and others of a blueish cast, with small fragments of shells inter- 

 mixed. 



At one place at Tallo Dalam, I found strata of a calcareous rock, laid com- 

 pletely bare, on the crest of a hill, dipping to the north-east, with an inclina- 

 tion of above 45°, and abruptly broken on the other side into a kind of stair. 

 I have a specimen of this rockf, which contains fragments of shells and fossil 

 wood. 



The appearance of unchanged and unfossilized masses of coral on the sur- 

 face of the hills, seems most readily explicable on the supposition, either of a 

 subsidence of the ocean below its original level, or a heaving up of the island 

 by a force from beneath. If we admit the former of these causes, indications 

 of a similar subsidence ought to be found on the adjacent coasts : but I am not 

 aware of any such having been observed. The great inclination of the strata, 

 and the dislocation they sometimes appear to have suffered, would seem to fa- 

 vour the latter hypothesis. It must still however be regarded as a phenomenon 

 of a most singular kind, that so large an island, diversified with numerous hills 

 from 800 to 3000 feet in height, should have been heaved up from the sea 

 with so little disturbance to the fragile marine productions on the surface. 



* It is highly deserving observation, that these rocks, particularly No. 19,099, have a striking 

 resemblance to parts of the green sand formation in England, especially to the rock called Kentish 

 rag ; and it is also worthy of remark, that in Sumatra, on the part opposite to Pulo Nias, at 

 Nattal-hill, a rock occurs, exactly corresponding to No. 19,099 : this bed, therefore, probably 

 extends across the channel that separates Pulo Nias from Sumatra. — Note by the Secretaries. 



+ This specimen. No. 19,103, is probably from one of the lower beds of the island; it bears a 

 considerable resemblance in its aspect to one of the rocks of the oolitic series. 



