192 Dr. Jack on the Geology and Topography 



filtration 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 col- 

 lected 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 pre- 

 sent elevation, the transition must have been effected with lit- 

 tle violence or disturbance to the marine productions at the sur- 

 face. The subjacent rocks are stratified ; among them I found 

 granular quartz, limestone, and calcareous sandstone * : to- 

 wards the southern end I also met with several kinds of lime- 

 stone, some a coarse yellowish-white stone, and others of a 

 blueish cast, with small fragments of shells intermixed. 



At one place at Tallo Dalam I found strata of a calcareous 

 rock, laid completely bare, on the crest of a hill, dipping to 

 the north-east, with an inclination of above 45", and abruptly 

 broken on the other side into a kind of stair. I have a spe- 

 cimen of this rockf, which contains fragments of shells and 

 fossil wood. 



The appearance of unchanged and unfossilized masses of 

 coral on the surface of the hills seems most readily explicable 



some of the oolitic beds of Europe; and there are dispersed through it faint 

 traces of coralline bodies, which probably indicate that it has been origi- 

 nally derived from corals ; but in general the substance of the coral has 

 been almost entirely dissolved, and the pores of what remains filled up by 

 sparry matter. This specimen has the aspect of an ancient rock. An- 

 other specimen, No. 19,104, exhibits very faint traces of coralline bodies; 

 but the rock is filled with innumerable irregular canties, which are stained 

 internally by oxide of iron : the specific gravity of this last is also that usual 

 to limestone. The rounded shape of the specimens, Nos. 19,104, 19,105, 

 19,106, leads us to infer tliat they were not procured from a rock in situ, 

 but were perhaps found lying on the surface, or imbedded in the soil of the 

 hills : if so, they may be considered as detritus ; and may therefore very 

 possibly be portions of solid strata existing in the island. The specimen of 

 recent coral, stated to be found in the hills, is water-worn, and appears to 

 have been a detached piece. — Note hy the Secretaries. 



* 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, there- 

 fore, probably extends across the channel that separates Pulo Nias from 

 Sumatra. — Note hy 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. 



on 



