DESCRIPTION OF THE CHIEF TYPES. 169 



corresponding increase of magnesia and alumina "amongst the con- 

 solidating oxides. The order of crystallization is thus : — 



(1) Magnetite .... FeO.Fe 2 = . 



(2) Pleonaste (Ceylonite) (Mg.Fe)O.(Al. Fe) 2 O s . 



(3) Spinel .... MgO.Al 2 3 . 



(b) Types rich in hornblende* 



In many places it is found that the non-felspathic types of 

 the charnockite series are characterised by the predominance of 

 hornblende over the other ferro-magnesian silicates, so that the rocks 

 would be more accurately described as pyroxene-amphibolites. But 

 as it seems certain from the evidence obtainable in places like 

 Tirrupur in the Coimbatore District, that the amphibole has in part 

 been formed by alteration of the pyroxene, it is better to recognise the 

 genetic relationships which these highly hornblendic types bear to 

 the purer pyroxenites than to separate them under the name amphi- 

 bolite, which would more correctly express their present minera- 

 logical composition. 



In one of the specimens (No. 9*317) obtained near the 

 Travellers' bungalow at Tirrupur, there are very pretty instances 

 Amphibolization of the illustrating the change from augite to horn- 

 Pyroxenes, blende ; patches of the latter mineral are 

 scattered through the large augites, showing, by their simultaneous 

 extinction, a crystallographic parallelism to one another, and by the 

 cleavage cracks, an axial parallelism also to the augite from which 

 they have been derived. Although this specimen contains consider- 

 able quantities of felspar, the ferro-magnesian silicates, augite, hyper- 

 sthene and hornblende, which it contains, are precisely similar in 

 character to the representatives of the same species forming the 

 associated ultra-basic masses. Near the Travellers' bungalow at 

 Tirrupur, narrow dykes or veins of a sparkling black amphibolite 

 (No. 9*309) are found ramifying amongst the granulitic rocks which 

 are composed, as already stated, of augite, hypersthene, hornblende 

 and felspar. These dykes of amphibolite bear to the felspathic rocks 

 E 2 ( 51 ;) 



