﻿444 ME. J. B. SCEIVENOE ON THE [Aug. I9IO, 



IV. Okigin op the Touemaline-Coettndtjm Kocks. 



It is hoped that the foregoing descriptions will give some idea of 

 the many varieties of the tourmaline- corundum rocks. That they 

 are not igneous is unquestionable. The prominence of the two 

 minerals tourmaline and corundum and the evidence of their 

 gradual development leave no room for doubt that they are 

 extraordinary examples of metamorphism effected by the tin-bearing 

 granite in beds the original characteristics of which cannot be 

 observed now in any sections showing a passage from the unaltered 

 to the altered state. That the granite and its modifications were 

 the cause of the metamorphism is evident from the geology of the 

 Kinta Valley, there being no other igneous rocks known in the 

 district ; and it is unnecessary to quote literature to show that both 

 tourmaline and corundum are already known as results of contact- 

 metamorphism by granitic rocks. 1 



Without sections such as those that I have just mentioned, namely 

 sections showing a passage from the unaltered to the altered rocks, 

 I doubt whether it is possible to prove anything regarding the origin 

 of these tourmaline-corundum rocks. The most that can be done is 

 to state the facts, and indicate the possible hypothesis to which they 

 tend. It may be that in future mining operations some section 

 will be laid bare, such as will confirm or refute the hypothesis which 

 I am about to put forward ; but, unfortunately, a knowledge of the 

 country and the mines of the present day makes me think it unlikely 

 that such a proof, or the reverse, will be obtained. 



Let it be admitted, then, that in the abundance of the minerals 

 tourmaline, corundum, white and brown mica, and spinel, we have 

 an example of metamorphism by a granitic mass that is remarkable 

 only in the degree to which these minerals have been produced. 

 There remains the problem of the origin of the extraordinary 

 bodies and cavities observable in the tourmaline-corundum rocks. 

 That structures of the same shape but different material existed 

 before the granite effected this striking transformation, and that 

 the spherical and other bodies and cavities were not created 

 de novo at the time of granitic intrusion, would be hard to question 

 under any circumstances, while the evidence of the bodies in the 

 light-coloured specimens under group (v) of the foregoing descrip- 

 tions (p. 440) places the matter beyond doubt. 



The points which seem to be most important in discussing the 

 origin of these structures are as follows : — 



(1) The rocks are all fine-grained. 



(2) When any ground-mass is visible apart from tourmaline and corundum, 

 it is very finely crystalline, and either siliceous or micaceous. 



(3) Carbonaceous schist is commonly found associated with the tourmaline- 

 corundum rocks. 



1 At Lenggong, in Upper Perak, pale-yellow tourmaline, corundum, and, 

 spinel were found together in crystalline limestone. All three occur in small 

 grains, the spinel appearing colourless under the microscope. 



