XXII. — On the Geology and Topography of the Island of Sumatra, and some 



of the adjacent Islands. 



By the late WILLIAM JACK, M.D. M.G.S. 



[Read April 18th, 1823.] 



X HE various journeys which have recently been made into the interior of 

 Sumatra,, have considerably extended our hitherto very imperfect knowledge 

 of the geography of that island, and have also furnished materials for a 

 slight outline of its geological structure. Some account of the information 

 which has been acquired on both these topics, may not prove unacceptable as 

 an accompaniment to a small collection of specimens of the rocks of that 

 country, which I have transmitted to the Geological Society. 



I obtained these chiefly from the western coast of the island, which, from 

 its greater boldness and proximity to the mountains, is the most accessible to 

 mineralogical investigation. It is to the eastern side that all the great rivers 

 take their course, where the extent of alluvial land is consequently much the 

 most considerable. If any reliance can be placed on native traditions, the 

 increase of surface on this side, by alluvial deposit, has been great and rapid ; 

 as in all their earliest histories the town of Palembang, which is now at least 

 sixty miles from the mouth of the river, is mentioned as a sea port ; and the 

 adjacent hill of Siguntangguntong as an island. The smoothness of the inland 

 seas, into which these rivers flow, must certainly favour the accumulation of 

 the earthy particles which they wash down. 



On the western side, a difference of character and aspect may be remarked 

 between that portion of the island which lies to the north of Indrapore and the 

 southern. The former comprises about two thirds of the length of the island, 

 and includes the richest and most interesting districts ; its coast is more irregu- 

 lar and broken, and is defended by innumerable small islands ; while the hills, 

 I at one time approaching towards the shore, at another receding from it, appear 

 to pursue no determinate line. In the southern portion, the coast is but slightly 

 indented, and is skirted by few islands; while the hills run in nearly a con- 

 I tinuous chain, as far at least as Bukit Pugong near Croee, at a distance of 

 ■ from ten to twenty miles from the coast, and form what is usually called the 

 Bukit Barisin or Barrier range. In the northern portion, there is much less 



