﻿176 



BK. W. K. JONES ON THE SECONDARY 



[vol. lxxii, 



(a) Weathering of Phyllites, etc., into Clays. 



At Kacha, Tambun, Lahat, and Papan the ' clays and boulder- 

 clays ' can be definitely traced in the tin-mines, from a structureless 

 clay through a clay showing traces of foliation, into partly-decom- 

 posed phyllites exhibiting distinct foliation, and occasionally con- 

 taining granitic intrusions which carry cassiterite and tourmaline. 



The remarkable rapidity with which rocks weather in tropical 

 countries is well known ; but, where the process is helped by heavy 

 evenly -distributed rainfall in a country covered with dense vegeta- 

 tion, it becomes so intense that within a few weeks freshly-exposed 

 surfaces of phyllites and shales are weatbered into a soft clay. 

 Where, however, phyllites and schists are heavily veined with silica 

 they become friable, but continue to preserve their structure in 

 places for a long period. 



Dr. J. W. Evans, in describing 1 the weathering of crystalline 

 rocks of the Malnad ('rainy area'), west of Mysore, states that 

 they have been converted down to a considerable depth into a 

 reddish or brownish clay-like material, which might be mistaken for 

 a Tertiary or Quaternary formation, were it not that it is traversed 

 occasionally by a quartz-reef which alone resisted the forces of 

 corrosion, and igneous rocks of the Nilgiri Hills are seen to weather 

 into a similar soft material. The same author describes the con- 

 version to a considerable depth of the Palaeozoic slates of Bolivia 

 into a deep-red or brown clay. 



Van Schelle 2 states that the argillaceous sandstone in the river 

 Bojan (Western Borneo) is weathered into clay to a depth of Si feet, 

 and is then indistinguishable in its upper layers from alluvial clay ; 

 and Th. Posewitz 3 wrote that 



' the weathering' of the eruptive rocks is also very great, a fact which was 

 impressed upon me during my ascent ef the mountain Pararawen in Southern 

 Borneo.' 



He found nothing but a completely decomposed rock, the nature 

 of which it was impossible to determine ; and even from a depth 

 of several feet he obtained only a very altered rock, which was 

 provisionally referred to granite. 



In the mica-schists of Fatoya (New Guinea), Prof. Lacroix 4 

 states that borings were pushed to a depth of about 200 feet 

 without reaching the fresh rock, the weathered rocks being unctuous 

 to the touch and friable when dry. 



Sections in which granite has been weathered into a soft friable 

 earth to a depth of 40 or 50 feet are very common in numerous 

 road-cuttings in Malaya, where small veins of silica and fracture- 

 planes can be seen continuous from the fresh to the weathered 

 rock. In some cases, as on the Pahang road, the rock is seen 



1 ' The Wearing-Down of the Rocks ' Proc. Greol. Assoc. vol. xxv (1914) p. 238. 



2 Neues Jahrb. vol. ii (1880) pp. 19 & 28. Quoted by Posewitz. 



3 ' Borneo ' transl. by P. H. Hatch, London, 1892. 



4 L. L. Fermor, ' The Work of Professor Lacroix on the Laterites of French 

 Guinea ' Geol. Mag. dec. 6, vol. ii (1915) pp. 36, 37. 



