TIN MINING IN PERAK. 21 
This is not to be considered a complete list, as small quantities of 
the ores of many other metals have also been found. 
The metalliferous ores in the alluvial beds are naturally derived 
from the older formations, but are in many cases much more commercially 
important, because they are more accessible and easier to work. In the 
formation of these beds, Nature has done on a large scale what a miner 
does in a small way: she has crushed and ground to powder vast masses 
of rock, and by the action of water has sorted out and concentrated 
within restricted areas all the valuable constituents. The agencies 
employed were the same as we see now at work. Briefly, they are water, 
air and heat. The surface of the rock having been softened by the 
combined action of these three powerful destroyers, the rain detaches 
fragments and carries them in the streams down the hillside. In their 
descent, being thrown violently against the rocks of the bed of-the stream, 
they chip off other fragments and themselves get gradually broken up, 
until on arrival at the level of the plains they are reduced to the state of 
sand and gravel. Here the heaviest particles deposit and the lighter 
gradually find their way far out on to the plain. This simple process 
continued for thousands of years wears away the hills and distributes 
their materials over the plains and at the bottom of the seas into which 
the waters ultimately flow. 
The alluvial deposits themselves are also subjected to a somewhat 
analagous process. The floor of a valley formed of a thick deposit of 
alluvium will in time be lowered by the action of the stream flowing 
through it, and thus the first deposited matter will all be again shifted 
and sorted. Streams and rivers do not usually lower the level of the 
whole valley equally ; thus it comes that portions of the old alluvium are 
frequently left at the sides of the valleys, forming what are called river 
terraces. The excessive rainfall of Perak does not favour the formation 
of these river terraces, or, it would be more correct to say, it rapidly 
rounds them off, and the numerous tributaries which come in at such 
short distances oa the hills on either side of the valleys cut them up 
and destroy their distinctive character; but still, in many localities they 
may be recognised. They have taken in the past a curious and interest- 
ing part in filling the caves of some of the limestone hills with tin-bear- 
ing drift. Some “of these caves are now over four hundred feet above the 
present level of the valley. They are worked for the tin contained in 
them; and the remnants of some of the river terraces are also mined to 
a considerable extent. 
From the foregoing it will be apparent that there are two distinct 
phases in the formation of alluvial valleys. Firstly, the filling in with the 
detritus of the hills, and secondly the sorting and partial carrying away 
of the deposit first formed. The two processes may be seen in operation 
sometimes in neighbouring valleys; and to a certain extent, in the same 
valley, at different seasons of the year. _ Flooding is an essential of the 
filling-in process, and variations of rainfall therefore affect it; but the 
alteration of the level of the lower part or outlet of a valley is the im- 
portant determining factor between the two pliases. The falling of a 
few trees, or the accumulation of some driftwood, will bank up a stream, 
and may cause it in a short time to deposit several feet of earth above 
