183.6,] 



Ncelgh.crries and Roondahs. 



wonderful chasm, the one to the right is granitic, having many boulders 

 of trap on the side and at the summit (No. 140) ; that to the left is 

 basalt, at least from the lower part of the Gap to the summit, decom- 

 posing into the yellow clay (No. 141). I say from the lewer part of 

 the Gap, because on the eastern side it overlays pegmatite. 



Not to interrupt the geological itinerary, I postpone describing this 

 extraordinary chasm, until the time arrives for giving an account of the 

 few excursions I made in the neighbourhood of Sispara, while at the 

 bead of the Malabar ghaut. 



The whole of the hills to the left of the Gap for a mile or two are 

 basaltic, so also those forming the left side of the ravine, along which 

 the road is cut. But after this distance, we come upon a very thick bed 

 of pegmatite, beautifully characteristic, underlaying the basalt of the 

 left hill of the Gap (No. 142). This pegmatite is very hard, and is 

 seen in the section for the road in the greatest perfection ; it contains 

 nothing else but milk-white felspar, and quartz of a faint aquamarine 

 colour, with a few nearly microscopic grains of hornblende, which, 

 when decomposed together with the rock, give to the porcelain earth a 

 motled appearance of ochreous specks. This rock continues with a 

 little interruption as far as the beginning of the descent to the Pio- 

 neer's Camp. But I must mention, before proceeding to the camp, a 

 most curious and at the same time very characteristic phenomenon, the 

 manner, namely, in which basalt has intruded into the granite of this 

 place. 



A mile or two before the descent towards Sispara, to the left of, and 

 between two and three miles from, the road, there is a high hill, having 

 a vertical, naked face towards the road ; between it and the hill a 

 ravine, overgrown with jungle, intervenes. In this precipitous facade, 

 which is quite exposed, even at this distance we discern a huge basaltic 

 dyke, which appears to have burst from the base of the hill, intruding 

 into the granite, giving out at the same time numerous ramifications, 

 to the left and right, which intersect the granite with many tortuous, 

 intricate and irregular diramations. The diagram (PI. 7, fig. 4) will give 

 better idea of the appearance of this intrusion than any description 

 can. These gigantic dykes are seen almost in every hill in this west- 

 ern boundary of the Koondahs. Unfortunately many of them (although 

 distinguished so clearly as not to leave the most distant doubt of their 

 nature) are inaccessible, on account of the impervious jungles which 

 surround them. This is the case with the last mentioned, and with 

 another nearer to Sispara, to the left of the road, in which the dyke 

 shoots up through the middle of the bluff facade of the hill, without 

 giving off any ramifications. The rocks from this place to Sispara are 

 granite, decomposing and decomposed. 



As I said that, during the few days wc remained at Sispara, I mado 



