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Mahakkam rivers and just beyond it, for a distance of 230 K.M. At 
that time nothing was known regarding the geological structure of the 
basin of the Upper Mahakkam, but a short time ago I was enabled 
to resume my studies on the distribution of these Radiolarites since 
there have been placed at my disposal the collections made by 
Nieuwenhuis 1 ) in the riverbasins of the Mahakkam and Upper Kajan 
in the years 1896—97 and 1898—4900, also the unpublished results 
of investigations made by Bonarelli along the rivers Kelai and Segah 
in the Berau district in the years 1905 and 1906, and finally a 
collection of rocks made by van Maarseveen along the river Kelai and 
its tributary the Keloh in the year 1907. From these studies it results 
that the Danau-formation extends eastward, without changing its 
character, from West Borneo into East Borneo right through the 
basin of the Upper Mahakkam; it reaches the watershed between 
the Mahakkam and the Kelai eastwards of the Boh-river, and is 
finally also found on the north side of this watershed in the basin 
of the Kfclai in the Berau district, always with the same strike from 
west to east or nearly so. At an average distance of 50 K.M. from 
the East coast of > Borneo it finally disappears in the Berau district 
under the younger, tertiary coast-range of East Borneo. 
The total length of the strip of country, in which the occurrence 
of the Radiolarian rocks has now been demonstrated, is 650 KM. 
and if the average width be taken at somewhat over 60 KM. this 
area occupies roughly 40000 square kilometers. The composition of 
the cherts and hornstones remains perfectly constant over the entire 
distance, and in every locality the same types of rocks are found 
again and again presenting the same macroscopical and microscopical 
characteristics. 
The first type is the Radiolarian hornstone, the true Radiolarite. 
This rock is semitransparent, hard, brittle and splintery and of a 
colour varying between milkwhite, red and greenish. It consists 
for about 97% of silica and is composed almost exclusively of 
closely packed tests of Radiolaria, joined together by a siliceous 
cement. The red-coloured varieties may be called Radiolarian 
jasper; they contain a small percentage of iron. The tests of t e 
Radiolaria are difficult to distinguish under the microscope | D * e 
milk-white varieties; they can be much better distinguished in t e 
red jaspery varieties, but best of all they are preserved and discern 
able in the greenish varieties of the Long Keloh in the Berau distric 
The second type is an argillaceous chert or siliceous clay-s ae > 
*) A. W. Nieuwenhuis. Quer durch Borneo. Leiden 1904. 
