﻿part 3] STANNIFEROUS DEPOSITS OF KINTA D1STKICT. 177 



weathered to a depth of 60 feet and more ; and, near Peretak, 

 in {Jlu Selangor, granite has been weathered in situ to a depth 

 exceeding 100 feet. 



The conversion, therefore, in a moist tropical climate, of rocks 

 such as schists, phyllites, and indurated shales into a fat clay, in a 

 valley where all the rivers have just flowed for many miles over a 

 decomposing granite, and the waters of which may be presumed 

 to be rich in alkalies, is not remarkable. 



Where the water continues to flow along the sloping junction of 

 the granite, that part of the beds which overlies the limestone 

 becomes converted into a soft structureless clay, and, helped by its 

 sinking movement on the gradually-dissolving limestone under- 

 neath, it loses its structure, whereas the overlying beds may still 

 preserve some of their structure. Such cases, which were recently 

 well illustrated in Siputeh, Lahat, and Serendah Valley, account 

 for the presence, in places, of a soft clay between the limestone 

 and the phyllites and schists. These clays in many cases, as, 

 for instance, at Tambun, Lahat, and Tronoh, have been definitely 

 traced into weathered phyllites showing foliation-planes (see 

 PI. XIV). 



Where the decomposed phyllites and indurated shales are veined 

 with granitic intrusions, mostly quartz- veins, the sinking move- 

 ment of such beds on the dissolving limestone breaks up the con- 

 tinuity of the veins, and results in a fine clay containing angular 

 boulders ; and, where the granitic intrusion was tin-bearing, the 

 ore preserves its angularity. A section illustrating this was par- 

 ticularly well exposed recently at Siputeh Mine. 



The conversion of such beds into clay, resulting, in some cases, 

 in landslips and soil-creep down steep slopes, also accounts for 

 angular boulders in a fine clay, and the process is common in 

 Malaya. 



The weathering process is particularly intense in alluvial deposits 

 where the waters, rich in alkalies, can percolate with ease through 

 the beds in various directions. At Kanching and other places in 

 Ulu Selangor, and at Gopeng, Kramat Pulai, Lahat, and many 

 other mines in Kinta, beautifully-rounded pebbles of different 

 types of granite have become so thoroughly decomposed that they 

 can be powdered into a fine sandy clay with ease between the 

 fingers. 



P. A. F. Penrose 1 observed the same phenomenon in the locality 

 which first suggested to Mr. Scrivenor the existence of glacial 

 deposits : — 



' Sometimes beds of coarse granite pebbles and boulders, forming the sub- 

 stratum of the tin-alluvium, have decayed in situ in the same manner as the 

 surface of the original rock ; and it is not uncommon to see rounded granitic 

 fragments converted into a soft putty-like mass, which, when broken up, 

 gives rise to angular particles of the original quartz of the rock and a soft- 

 clay resulting from the decay of the felspar. Hence angular quartz may 

 often be found in deposits that have been transported long distances. Such an 



1 Journ. Geol. Chicago, vol. xi (1903) p. 143. 



