C/2 On the Local and Relative Geology of Singapore, [July, 



pervades the rooks, and the seams have either a thin plate of blackish 

 ferruginous crust included between them, or their sides have a similar 

 thin coating which is often covered with an exceedingly minute mamil- 

 lation. In some cases the matter between the seams or ferruginous 

 walls, has been dissipated, and the rock appears as a black honeycomb. 

 In all instances of high calcination the sandstone is greatly indurated. 

 It is sometimes converted into a crystalline rock. 



Friable shales, again, are sometimes changed into a dry powdery 

 matter resembling volcanic ash. 



Where the bodies of the strata are not altered their planes of junc- 

 tion are sometimes slightly indurated and mamillated. The gas in 

 every case has taken the readiest channels to the surface, — and where 

 fissures have not assisted its emission, it has forced itself through the 

 planes of least cohesion, such as the junction planes of different beds, 

 cleavage planes, &c. It thus often exposes the internal structure of the 

 rock where it would otherwise appear compact. The composition of 

 the rock has often had a great influence in determining the channel of 

 emission, so that its action sometimes is chiefly confined to one or more 

 strata, the adjoining beds appearing to be little if at all affected. 



Quartz frequently accompanies the ferruginous change, but rarely to 

 a considerable extent. 



The above are the most common modes of alteration, but there are 

 others approaching nearer to true metamorphism. Clay is converted 

 into a porcellainous or jaspideous substance, — sandstone into a hard 

 siliceous flinty substance. Conglomerates and breccias have frequently a 

 base of this nature.* 



The mechanical force accompanying the evolution of the hot ferrugin- 

 ous gases or vapours has been great, but it has been exerted within 

 narrow limits. Thus the strata are often vertical, and generally rise at 

 high angles, but the dip varies much, and even in adjacent hills of the 

 same connected range is sometimes reversed. Yet they are never 

 raised more than a few hundred feet above the common basal level, and 

 the majority of the almost innumerable hillocks which compose the 

 ranges of Singapore, are probably rather under than above 100 feet. 



* I have since found on the eastern coast ofPulo Krimun Kichi (the Little Carimon) 

 great masses of clays and conglomerates transformed into a perfect crystalline chert as 

 hard as flint.— J. R. L. 



