420 



expedition to tiio islands of the Timor group in J910— 1912 in 

 triassic and Jurassic deep-sea deposits, on the Island of Timor, and 

 also well developed in similar Jurassic deposits on the Island of Rotti, 

 and pievioiisjj, (in J 894, and later) I had noticed them in abysmal 

 deposits of the precretaceous probably Jurassic Danaii formation, 

 occurring in West and East Borneo. 



Rock specimens were collected by the undermentioned observers, 

 and their examination has aff'orded proof from which several deduc- 

 tions have been inchided in this paper. 



a. In 1894 in Central Borneo by the autlior. 



b. In 1898—1900 in the basin of the Maiiakkam River by Prof. A. 



W, NiEUWENHUIS. 



c. In 1902 in the Long Keloh, a small branch of the Long Kelai, 

 which is one of the great tributaries of the Berau-stream in East- 

 Borneo, by Mr. van Maarsevken, 



d. in 1911, on the island of Timor by the Netherlands Timor- 

 expedition led by the author, 



e. in 1911 and 1912, on the island of Rotti, by Dr. H. A. Brouwer, 

 one of the members of tlie same expedition. 



All the specimens collected with tlie exception only of these of 

 Central-Borneo are stored in the geological museum of the Technical 

 Highschool at Delft. 



On the mode of occurrence of the manganese in the rocks. 



In the rocks just mentioned the manganese has been concentrated 

 in various ways : 



1. As grains, i.e. minute concretions, frequently only recognisable 

 as such under the microscope, occurring throughout the rock. This 

 form of concentration is very common in red shales, which are the 

 equivalent of recent red clays. These shales vary in colour from 

 brick red to chocolate brown, they invariably include a noticeable 

 proportion of silica, (in places a little lime) and, in varying quantities, 

 tests of radiolaria. The characteristic red colouration is due to the 

 presence of oxide of iron, and this tint deepens into chocolate brown 

 in proportion to the increase in the percentage of manganese, 

 entering into the composition of the rock. 



This mode of accumulation is of almost general occurrence in 

 all deep-sea deposits containing much clay '), but is of less import- 



1) In modern deep-sea deposits the bulk of the manganese is just as well con- 

 centrated in small grains, causing the brownish red an! chocolate brown colour 

 of tlie deep sea silt, esjiecially of the redclay of the Pacific and the Indian Ocean. 

 Compare J. Murray and A. V. Rfnard 1. c. p. 191, p. 3il and PI. XXII fig. 1. 



