1847.] including Notices of Sumatra, the Malay Peninsula, §c. 6/1 



rock has been merely impregnated with hot ferruginous gases or vapours 

 — where the calcination has not been great the original structure of 

 the rock is better preserved than in the merely impregnated rock, 

 because, in the latter, the indurating action of the iron, the different 

 degrees of its oxidation when it comes within the influence of water 

 and air, and the washing out of the softer portions in the hollows, 

 often give it an amygdaloidal or vesicular structure totally different 

 from that of the original rock — a slight roasting on the other hand 

 preserves the latter and saves it from meteoric destruction. The limit 

 of this preserving power is soon reached, and every higher degree of 

 heat and larger infusion of iron exerts in each rock, a corresponding 

 destructive or altering power, and approaches nearer that point where 

 the original differences in the rocks cease to be distinguishable. The 

 extreme limits of this class appear to be where the rock is merely 

 scorched on the surface, preserving its original character beneath, and 

 where it is thoroughly reduced to a cinder. This class of rocks very 

 frequently presents mamillated and botryoidal surfaces. It occurs 

 in dykes, and on the sides of fissures through which hot blasts appear 

 to have rushed. It also occurs in an outer layer or thick crust over 

 rocks of the 5 th class, in which case it would appear that the different 

 effects produced by the same gas arose from the upper crust being 

 exposed to the air and consequently burnt. In the same way the 

 calcination to some depth on the sides of fissures may have arisen in 

 certain cases, not from the gas that rushed through them being hotter 

 than that in the body of the rock (though this was most likely the fact 

 in general) but from the presence of air producing combustion. Be- 

 tween dykes of this last class rocks altered in the above 5th degree are 

 common — but dykes of the 5th degree also occur. The difference in 

 every case will depend on the relative intensity of the heat and degree 

 of ferrugination of the gas, and the fact whether there was air to support 

 combustion or not. 



The preceding remarks are applicable chiefly to rocks either compos- 

 ed of clay or in which there is a basis of clay. But a very small pro- 

 portion of clay suffices for the exhibition of the above modes of action. 

 When the rock is wholly arenaceous, nodules are not formed. The 

 rock is reduced to a dry incoherent or friable mass where the action 

 has been slight. Where it has been greater, a net work of cracks 



4 s 



