398 Dr. Jack, on the Geology and Topography 



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 Malayan peninsula, its parts b}' no means exhibit a cor- 

 responding regularity. In this particular the greater number of maps convey 

 an erroneous idea. 



It has been ascertained, that the Poggy islands, Pulo Nias, and the whole 

 of the northern coast, are laid down in Horsburg's charts considerably to the 

 westward of their true position : a circumstance which materially affects the 

 supposed breadth and form of the island ; and it is singular that, with the 

 exception of Acheen Head, Bencoolen, and perhaps Flat Point, there is 

 scarcely a place on the west coast (where we have had establishments for 

 above a century) of which either the latitude or longitude is exactly deter- 

 mined. The mountains, for the most part, lie nearer to the western coast 

 than might be supposed from Marsden's map ; and a greater length of course 

 must, therefore, be given to the great eastern rivers, which have their sources 

 among them. It has been an object of interest in our late investigations to 

 trace more correctly the course, the relative position, and true sources of these 

 noble streams, most of which afford safe navigation to the largest vessels for 

 upwards of a hundred miles above their mouths. 



The basis of the island of Sumatra is probably primitive ; granite has been 

 found in Menang kabau and at Ayer Bangy : but trap rocks are perhaps the 

 most widely diffused ; while the mountains of greatest elevation, and which 

 stand in some degree insulated, are generally volcanic. The volcanoes of 

 Sumatra have somewhat a difl'erent character from those of Java : the former 

 generally terminating at the summit in a ridge or crest; while the latter are 

 more exactly conical, and have for the most part much broader bases. 



Commencing at the north with Acheen [respecting which the mission of 

 Sir T. S. Raffles in 1819 afforded the means of adding to our information] I 

 have merely to remark, that the mountains which terminate in Acheen Head, 

 together with the adjacent island of Pulo Way, and the coast to the eastward 

 including Pedier, are of calcareous formation. 



Proceeding southwards along the western coast, we come to the Bay of 

 Tappanooly, which forms a large and deep indentation among the hills of the 

 Batta country. In these hills, which come directly down to the margin of 

 the sea, as well as in the small islands within the circumference of the bay, 

 the rocks consist chiefly of a fine-grained sandstone, frequently striped with 

 various shades of yellow and red. The strata are in general even and regu- 

 lar, and but slightly inclined, occasionally, however, exhibiting partial disturb- 

 ances and undulations. 



