534 On the Local and Relative Geology of Singapore, [June, 



nature, at one time producing a superficial effect, either uniform in its 

 character, or small in degree, and at another time increasing in violence, 

 and at particular points causing convulsive elevations of the rocks in 

 the form of hills, frequently in undulating ridges and chains, the linear 

 directions of which were, it may be, determined by a pre-imposed ten- 

 dency to fracture, as will be noticed in the sequel. This force was 

 apparently of a volcanic, or what, to distinguish it from concentrated 

 well developed volcanic action, may he called a semi-volcanic nature, 

 producing great heat at particular places, which sometimes merely 

 indurated or calcined the softer strata and reddened the superjacent 

 soil, but often in steam or gases, and occasionally in mud or semi-fused 

 rock burst through them, or found a vent in fissures caused hy ruptures 

 during the process of elevation. "When the heat was most intense, 

 fused rocks or semi-fused fragments were cast up through these vents. 

 As its intensity decreased fragments less altered and masses of clay and 

 sand were ejected. The volcanic steam, gases, or fluids were charged 

 with iron which left strong marks of its presence wherever these were 

 most active, rendering most of the fused and semi-fused rocks, in dykes 

 or ejected above the surface, highly ferruginous and impregnating all 

 the softer adjacent rocks. 



In some places the force, although of unusual violence, was at the 

 surface chiefly mechanical, rending solid sandstones and tossing up 

 and mingling the fragments with masses of soft clays and shales. 

 Thus on some parts of government hill and the adjoining hill (Mt. 

 Sophia) large angular blocks of solid sandstone, some from 600 to 800 

 cubic feet in bulk, are found at the surface and at various depths be- 

 neath it in a confused mass of clays and shales. In the same hills howe- 

 ver there were also subsequently formed volcanic fissures, through which 

 torrified rocks were ejected into the air and strewed over the surface so 

 as in some places to form a thick bed over the disrupted sandstone, &c. 



This extreme degree of local mechanical violence unaccompanied by 

 simultaneous igneous action reaching the surface, is, however, rare, and 

 may have been in some measure caused by a greater thickness and 

 compactness in the resisting rock. But in general the upheaving of 

 the hills has been attended with a violent agitation or tremor, producing 

 the phenomena alluded to in a former page as due to concussion. ' 



From what has been said it will be seen that the volcanic forces were 



