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Geology of the Neilgherries. [No. 12, new SERIES. 



garnets diffused extensively These granites were projected up 

 the funnels in the syenite, and filled the basins above. 



And thus we might account for the occurrence of the syenite in 

 the positions denoted in figure 1st, a — a being syenite, and b — b 

 granite. Pegmatite is found constantly on the Neilgherries cap- 

 ping syenite hills as here represented. 



I now arrive at that period in the geological history of these 

 hills, marked by the projection of the trap -rocks. These were 

 injected by subterranean forces into the cracks and fissures that 

 water, converted into steam by the subjacent heat, had created in 

 the rocks in its efforts to find vent. Some of the fissures however 

 were not filled, and may still be seen empty. 



First, amongst the trap-rocks or overlying, I class the red iron 

 ore, carelessly designated laterite by some of our geologists, and 

 which I have ascertained to be a combination of silica, alumina, 

 water, and peroxide of iron. This rock is most extensively dis- 

 seminated on these hills, appearing 'in overlying masses capping 

 the highest peaks in dykes traversing their sides in veins that 

 have overflowed their elevated vents and escaped down the sides 

 of the hills, and in low rounded hills, all the result of a discharge 

 from below. 



Iron enters largely into the composition of the other trap-rocks 

 of the Neilgherries, and it is worthy of observation that in the 

 three great epochs of this history it is found extensively prevail- 

 ing. In the first period, when the syenite was elevated, iron is 

 found disseminated throughout the different varieties of this rock, 

 combining with silica, magnesia, lime and alumina, to form the 

 hornblende of it. In the second period, at the time of the pro- 

 jection of the granites, we find it in connexion with other minerals 

 entering into the composition of the garnet family, abundant on 

 these hills : cinnamon stone, idocrase, almandine and common 

 garnets, being very frequent. In the trap epoch, iron appears as 

 the red iron ore, a decided trap-rock, and it occurs under different 

 forms as veins traversing the whole series. For a more detailed 

 account of the mineral forms of this metal, I refer the reader to 

 my " additions to the mineralogy and geology of the Neilgherries." 



