|836.] betiveen Madras and the Neilgherries. 2.5 



derlaying rock in all this district. Some of the projecting rocks assume 

 the appearance of mica slate, for the reason so often mentioned. 



Proceeding towards MacDonald's choultry, we see some small ridges 

 of white quartz rock, which are distinguishable even from a distance, 

 on account of their colour, different from the greyish hue of the gneiss, 

 and the black of the hornblende slate. They are the outgoings of th$ 

 beds of quartz in the gneiss (No. 87). 



Although the last mentioned rock is seen as subjacent to the horn- 

 blende slate, still the diorite continues to form all the hillocks and 

 eminences, on both sides of the road. This rock is clearly seen, eight 

 miles west of MacDonald's choultry, as a subordinate rock to the gneiss, 

 in a deep nullah near Conjamallee hill, where the gneiss occupies the 

 lowest situation in it, while the green-stone slate forms the whole hill. 



The hornblende slate formation terminates, or at least is interrupted^ 

 about ten miles west of MacDonald's choultry, and is succeeded by 

 a granitic rock, or, more properly, by protogine, containing, besides 

 felspar and quartz, some plates of talc (No. 88). They say that in 

 this rock occasionally aqua-marine is found. 



By far the greatest number of the masses along the road are porphy* 

 ritic in structure, or real porphyritic granite ; but in those blocks, in 

 which the mica is disseminated uniformly through the rock, and the 

 felspar in granular pieces, it assumes the aspect of granite (No. 89). 



For miles before reaching Sanklydroog, we clearly see that the 

 hills we are approaching, are of a different formation to those we have 

 just left ; to the tame, rounded, blackish outlines of the hornblende 

 hills, covered with thick, shrubby, and arboreous vegetation, we see 

 succeed the abrupt, precipitous, naked and whitish appearance of those 

 we are approaching. 



Sanklydroog. — In all the neighbourhood of Sanklydroog, the ele- 

 vated places, such as hillocks and rocky clusters, are of the porphyry 

 we are going to describe ; but in the ravines, nullahs, water-courses, 

 and, in general, in the plains, the protruding rock is gneiss (No. 90). 

 It forms the lower skirts of many of the hills east of the Fort ; and 

 even in the declivities of some of them, the gneiss is seen traversed by 

 veins or dykes of porphyry. In this gneiss the mica in strata is as fre- 

 quent, as in the places often mentioned in these Notes. 



This porphyry is composed of large crystals of pale flesh-coloured 

 felspar, imbedded in a paste of the same mineral in the compact state, 

 and of the same colour as that of the crystals. Some of the masses 

 have hardly any imbedding material, but are an assemblage of frag- 

 ments of crystals confusedly mixed together. This rock contains 

 neither mica nor quartz (No, 91). 



