190 Notes Illustrative of the 



necessary for the formation of the laterite had collected 

 there and a small nest of it formed. The circumstances 

 of the case excluded from consideration both the aqueous 

 and igneous causes, whose aid has usually been called in 

 to explain the origin of laterite, and the clear evidence of 

 its being due to the slow but certain operation of atmos- 

 pheric agents amounted to demonstration. The bewilder- 

 ing effect of a multiplicity of contending opinions gives to 

 such simple yet well marked observations on the point at 

 issue, additional value and interest. 



As the resultants of decomposition, it is natural to ex- 

 pect that the character and composition of both laterite 

 and lithomarge will be very variable, since the constitu- 

 tion of the parent rocks will necessarily influence that of 

 the derivatives. Hence it has been found by observation 

 that those rocks in which the proportion of quartz to 

 felspar and hornblende is considerable, decompose into 

 laterite, while those in which quartz is less abundant usual- 

 ly exhibit the lithomargic character. The same specimen 

 sometimes presents both of these formations, the one pass- 

 ing imperceptibly into the other and clearly establishing 

 the position that they are effects of the same cause modi- 

 fied by concomitant circumstances. These circumstances 

 are externally the variations of heat and moisture, and 

 internally the greater or less degree in which the consti- 

 tuents of the rocks predispose them to be affected by 

 the oxidising influence of the atmosphere. Throughout 

 nearly the whole of Southern India sienite prevails, and 

 with it laterite is very generally associated. But sienite 

 contains hornblende as one of its principal ingredients, 

 and the large quantity of oxide of iron this contains pe- 

 culiarly disposes it to yield to atmospheric action. Hence 

 then follows the disentegration of the mass; the felspar 

 furnishes the alumina necessary for the matrix, and the 

 quartz resisting decomposition, and becoming ferrugin- 

 ous from the infiltration of the iron, forms the imbed- 



