26 



whole to arrange itself in false beds coinciding with the surface of the erupted mass. At 

 another place he observed a similar alternation of beds, which , however, in this case ap- 

 peared to be at right angles to the hearing of the granitic mass in which they occurred. 

 Schorl rock and granite were seen in anolher locality in alternate zones. These phenomena 

 appear to be similar to some of those of Pulo Ubin which we are considering. 



Whatever were the mechanica! conditions under which the mass of Pulo Ubin solidified 

 from a fluid state, we must admit that to no single uniform causes can we ascribe tbe phe- 

 nomena which it presents. Assuming, as I think observalion requires , that the whole is of 

 one contemporaneous origin, we must allow that the chemical ingredients of the mass were 

 irregularly distributed, if not originally, then at a time immediately preceeding solidifica- 

 tion. Variable mechanical disturbance may have been mainly instrumental in producing local 

 inequalities in chemical and polar action during the transition from a fluid to a solid state, 

 and this might have the effect of attracting certain elements to particular places and there 

 exciling particular mineralogical developments. A portion of the rocks beneath which the 

 granite rosé may have been melted into it, and varied the relative proportions of the old 

 ingredients, or added new ones. It is perhaps rash to pursue this subject without an exact 

 chemical analysis of the rocks under consideration. But so far as we can take the ascertain- 

 ed gencral composition of minerals as a guide, we may enquire whether the passage of the 

 same connected mass from a rock in which mica is entirely absent into one in which it 

 predominates does not support the above views ? Can the frequent substitution of hornblende 

 for mica be otherwise explained ? It is trüe there is a great similarity in the chemical com- 

 position of the different minerals which make up the rocks of the Island, but some power- 

 ful forces must have operated to cause the segregation , in limited spaces, of certain elements 

 from the surrounding fluid mass. Mica is very inconstant in the proportion of i(s ingre- 

 dients, and there are species which approach very closely to hornblende, but even in these 

 the lime of the lattcr is represented by potash. Where we find the mica ceasing, and horn- 

 blende taking ils place, we must suppose that in the original fluid or viscid mass lime had 

 been segregated in the space now occupied by the latter. 



A circumstance mentioned by Sir H. de la Beghe with respect to the granite of Devon 

 and Cornioall bears materially on this subject. He says that towards the borders of the gra- 

 nitic tracts, — that is where the granite comes in contact with the circumjacent rocks, — 

 it alters its character, passing from a compound of quartz, felspar and mica to a schorla- 

 ceous rock. The mica first disappears, being replaced by schorl, and then commonly the 

 felspar also disappears, and the rock is converted into a compound of quartz and schorl 

 which are generally aggregaled in nearly equal proportions. Sir H. de ia Beghe also men- 

 tions that near the joints by which the granite is tra versed, its character alters, and that it is 

 adjoining these joints that the character of the mass at its confines becomes particularly 

 quartzose and schorlaceous. In the lower spurs of a granite chain in Pinang I remarked 

 that the rock was devoid of that general uniformity throughout considerable tracts which 

 distinguished the central ridge. It was variable in its composition and texture, being fre- 

 quently coarse grained. Quartz was largely developed, and in some places schorl, which I 

 do not recollect having any where noticed in the ridge, appeared in large crystals and fi- 



