333 



Geology of the South of India. 



[Oct. 



nately, I have no type with which to compare it, as in the case of 

 the basalt of the Giant's Causeway. Its colour varies with its con- 

 stituents, but is most usually gray. It is easily frangible, very fre- 

 quently friable, and is almost always porous and anivplaloidal. It 

 appears to be composed of earthy felspar and hornblende, with a 

 considerable proportion of oxide of iron. It is always easily fusible 

 into a black scoria, or glass, according to the quantity of zeolite 

 which it contains : of all the trap-rocks, it abounds the most in sim- 

 ple minerals : They are — Quartz." 



'* Calcedony and calcedonic agates, enclosing crystals 

 of carbonate of Vime. 



Common and semi-opal. 



Heliotrope. 



Plasma, or translucent heliotrope. 



Sldbite. 



Analcime. 



Natrolite. 



Icthyopthalmite. 



Felspar. 



Carbonate of lime and green earth." 

 *' I have never been able to discover in it either augite or hornblende 

 in distinct crystals. When the surface of the land is strewed with 

 these minerals, it is a certain indication, that the rock beneath is 

 wacken. With regard to the situation of this rock, 1 have rarely 

 seen it on the summits of hills, but much more frequently at their 

 bases, and forming the flat, elevated plains. I shall have occasion 

 to advert to this rock again, when I proceed to describe the petrifi- 

 ed shells." 



**The nodular basalt is, perhaps,the most common form of trap in 

 this mountain range, as well as in other parts of India. It more 

 commonly forms the surface than either of the rocks, and is as fre- 

 quently seen on the summits, as it is at the bases of the mountains. 

 It rarely abounds in minerals of any kind. It is the principal source 

 of the rich, black diluvian soil of India, commonly called black cot- 

 ton soil. I have little to add to the former description of it. Its ex- 

 ternal structure is sometimes beautifully developed by decompositi- 

 on, since, in a mass of about six inches diameter, it is possible to 

 count above twelve concentric layers, and on striking the nucleus a 

 slight blow with a hammer, one or two more layers are broken off. 

 It is owing to this facility of decomposition, that the annual rains 

 carry down such vast quantities of alluvial soil from its surface, 



