2 
1150 
centre; in fact in Sankulirang the deposits become more and more 
pelagic, as the sediments lie more eastward, and point to an old 
marine territory in Macassar Strait (L. Rurren, l.c. 1916). It does 
not seem probable, though, that also the western geosyncline was 
bounded in the N.W. by a deep sea. First of all there are factors 
pointing to the smaller significance of this geosynclinal territory 
than that of the East. The strike of the western geosyncline almost 
coincided with the northwest coast of the island as is shown on the 
map of C. Scumipr (le). In its south-western elongation nothing is 
known of a continuation of the subsided area; we find ourselves 
there in the old landmasses of Sambas') and the Natuna-Islands ’). 
One is impressed with the idea that the subsided region, which 
most likely extended from the Philippines as far as the northwest 
coast of Borneo, terminated here. There is still something else. In 
1914 1 obtained from Dr. NieTHAMMER a fragment of limestone from 
a territory far removed from the Borneo-coast viz. the islet of Man- 
galum (see sketchmap). It is an Operculina limestone, which, it is 
true, includes only Operculina complanata Defr., and which on that 
account may be quaternary as well as tertiary, but it bears a close 
resemblance to a Lepidocyclina-bearing Operculina-limestone from 
Pulu Labuansklambu near the northwestern point of Borneo. It cannot 
be doubted, therefore, but that the the limestone from Mangalum is 
still met with in Jitoral facies so far from the coast, so that it seems 
highly improbable that the Northwestern geosyncline should have 
been bounded in the North-West by a deep sea. It may be deemed 
more probable that here lay a subsided area, which at one time 
was alternately shallow sea, delta-territory or low land; that it was 
bounded on the one side by the old land-centre of Borneo, on the 
other by an old continent now transgressed by the Chinese Sea, 
and formerly perhaps connected with Indo-China, which is also an 
old continental region. According to this view both J. Morey and 
MOLENGRAAFF would be right, the former in referring the source of 
the material of the tertiary formations of Northwest Borneo to the 
Northwest, the latter in looking for it in the South-east. Moreover 
this view would also favour the conclusion that the central landmass 
of Borneo, which had already to contribute so much detritus towards 
the East and the South-East, was somewhat disburdened as to its 
contribution towards the Northwest. 
Utrecht, 2 Febr 1921. 
1) N. Wine Easton. Versl. Geol. Sectie. Geol. Mijnbk. Gen. I. 1914. p. 179—189. 
2) P G. Krause. Samml. Geol. Reichsmus. Leiden. (1) V. 1898. p. 221—236. 
