[ 18.5 ] 



XXX. On the Geology arid Tojyography of the Island of Su" 

 matra, and some of the adjacent Islands. By the late 

 William Jack, M.D. M.G.S* 



nPHE 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 I'ocks of that country, which I have trans- 

 mitted to the Geological Society. 



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

 which, from its greater boldness and proximity to the moun- 

 tains, 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 de- 

 posit, has been great and rapid ; as in all their earliest his- 

 tories the town of Palembang, which is now at least sixty miles 

 from the mouth of the rivei', 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 

 compri.^es about two-thirds of the length of the island, and in- 

 cludes the richest and most interesting districts; its coast is 

 more irregular and broken, and is defended by innumerable 

 small islands ; while the hills, at one time approaching to- 

 wards the shore, at another receding from it, appear to pur- 

 sue 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 continuous chain, as far at least as Bukit 

 Pugong near Croee, at a distjuice of from ten to twenty miles 

 from the coast, and form what is usually called the Bukit Ba- 

 risin or Barrier range. In the northern portion there is much 

 less appearance of parallelism in the distribution of the hills; 

 and though the whole of Sumatra may, in a general view, be 

 considered as forming a great chain parallel to that of the 



• From the Geological Transuctions, New Scries, vol. i. Part II. p. t397- 

 VnlGr,. So. 323. March ]H2r>. A a Malayan 



